In a Sane World, This Would Be the Top Story – Part I

Climate Update

What have we learned since the IPCC’s 5th Assessment Report?

Adapted from Royal Society’s November 27, 2017 Climate change update report
Abbreviations used throughout:

  • IPCC – Intergovernmental Panel on Climate Change
  • AR4/AR5/AR6 – Fourth/Fifth/Sixth Assessment Report of the IPCC
  • RPC – Representative Concentration Pathway

Introduction

  1. Question 1  – How sensitive is global temperature to increasing greenhouse gases?
  2. Question 2 – How are methane concentrations changing and what does this mean for the climate?
  3. Question 3  – Was there a ‘pause’ in global warming?
  4. Question 4 – How high could sea level rise because of anthropogenic climate change?
  5. Question 5 – Decreasing Arctic sea ice – is there any influence on the weather in middle latitudes?
  6. Question 6 – Have temperature and rainfall extremes changed, and how will they change in the future?
  7. Question 7 – Are there thresholds beyond which particularly dangerous or irreversible changes may occur?

Introduction

“Climate change is one of the defining issues of our time.” – Dr. Ralph J Cicerone and Sir Paul Nurse, in the foreword to ‘Climate Change: Evidence and Causes. An overview from the Royal Society and the US National Academy of Sciences’ 2014

Climate has a huge influence on the way we live. For example, it affects the crops we can grow and the diseases we might encounter in particular locations. It also determines the physical infrastructure we need to build to survive comfortably in the face of extremes of heat, cold, drought and flood.

Human emissions of carbon dioxide and other greenhouse gases have changed the composition of the atmosphere over the last two centuries. This is expected to take Earth’s climate out of the relatively stable range that has characterized the last few thousand years, during which human society has emerged.

Measurements of ice cores and sea-floor sediments show that the current concentration of carbon dioxide, at just over 400 parts per million, has not been experienced for at least three million years. This causes more of the heat from the Sun to be retained on Earth, warming the atmosphere and ocean.

The global average of atmospheric temperature has so far risen by about 1 ̊C compared to the late 19th century, with further increases expected dependent on the trajectory of carbon dioxide emissions in the next few decades.

In 2013 and 2014 the Intergovernmental Panel on Climate Change (IPCC) published its fifth assessment report (AR5) assessing the evidence about climate change and its impacts. This assessment considered data from observations and records of the past. It then assessed future changes and impacts based on various scenarios for emissions of greenhouse gases and other anthropogenic factors. In 2015, almost every nation in the world agreed (in the so-called Paris Agreement) to the challenging goal of keeping global average warming to well below 2°C above pre-industrial temperatures while pursuing efforts to limit it to 1.5°C.

With the next assessment report (AR6) not due until 2022, it is timely to consider how evidence presented since the publication of AR5 affects the assessments made then.

The Earth’s climate is a complex system. To understand it, and the impact that climate change will have, requires many different kinds of study. Climate science consists of theory, observation and modelling.

Theory begins with well-established scientific principles, seeks to understand processes occurring over a range of spatial and temporal scales and provides the basis for models. Observation includes long time series of careful measurements, recent data from satellites, and studies of past climate using archives such as tree rings, ice cores and marine sediments. It also encompasses laboratory and field experiments designed to test and enhance understanding of processes. Computer models of the Earth climate system use theory calibrated and validated by the observations, to calculate the result of future changes.

There are nevertheless uncertainties in estimating future climate. Firstly the course of climate change is dependent on what socioeconomic, political and energy paths society takes. Secondly there remain inevitable uncertainties induced for example by variability in the interactions between different parts of the Earth system and by processes, such as cloud formation, that occur at too small a scale to incorporate precisely in global models.

Assessments such as those of the IPCC describe the state of knowledge at a particular time, and also highlight areas where more research is needed. We are still exploring and improving our understanding of many of the processes within the climate system, but, on the whole, new research confirms the main ideas underpinning climate research, while refining knowledge, so as to reduce the uncertainty in the magnitude and extent of crucial impacts.

 

Figure 1 – Historic atmospheric carbon dioxide levels (NOAA)

 

This report considers a number of topics that have been a focus of recent attention or where there is significant new evidence. This is by no means a comprehensive review such as that being carried out for the AR6 or in IPCC special reports that are underway. It instead tries to answer, in an authoritative but accessible way, some of the questions that are asked of climate scientists by policymakers and the public. The answers start from the evidence in AR5, updated by expert knowledge and by a necessarily limited assessment of work published since then.

A full description of the process used is discussed in the appendix. The information here is supported by supplementary evidence available on the Royal Society webpages (royalsociety.org/climatechange) that describes the evidence base and literature sources used. This report does not attempt to cover every topic, and does not address more distant socioeconomic impacts of climate change such as its possible impact on migration and conflict. In particular, it does not discuss policy questions about how the aims of the Paris climate agreement might be achieved.

Each section of this report is designed to be read on its own, but the document as a whole follows a broad thematic progression, starting with aspects relating to the physical basis of climate change, and progressing through physical impacts towards those related to ecosystems and human wellbeing. The report shows where new studies are starting to fill identified gaps in knowledge. In some cases, new work suggests changes in the probability of certain outcomes occurring, but in most cases the broad statements made by IPCC still appear valid.

QUESTION ONE – How sensitive is global temperature to increasing greenhouse gases?

Summary

In 2013 the IPCC report stated that a doubling of pre-industrial carbon dioxide concentrations would likely produce a long-term warming effect of 1.5 – 4.5 degree Celsius; the lowest end of that range now seems less likely.

In AR5 IPCC said:

Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C.

Transient climate response is likely in the range 1.0°C to 2.5°C.

What is this about?

Climate sensitivity is a measure of how global surface temperature rises in response to increasing atmospheric concentrations of greenhouse gases. Understanding this measure provides insight into the amount of carbon that can be emitted for a given amount of future warming.

A higher value of sensitivity implies a lower remaining budget of greenhouse gas emissions to stay below a given warming threshold, and vice versa.

Equilibrium climate sensitivity is the increase in global surface temperature that would arise from the Earth fully adjusting to a doubling of atmospheric carbon dioxide (generally calculated from its preindustrial level). Temperature adjustment is slow, and surface temperatures will continue to rise well after the date of the doubling (even if the concentration of carbon dioxide has then stabilised).

In contrast, transient climate response is the increase in global surface temperature at the time when doubling of carbon dioxide occurs and relates more directly to the temperature increases we might expect to see in the coming century. The transient response represents a situation in which the climate has not yet fully adjusted and so is smaller than the equilibrium sensitivity.

The heat-trapping properties of carbon dioxide have been known since the 1860s and, if the only thing to change was the carbon dioxide level, it would be straightforward to calculate the warming resulting from a given concentration.

However, physical processes, known as climate change feedbacks (due, for example, to changes in humidity, cloud or ice cover) modify the direct impact of carbon dioxide substantially.

Climate sensitivity can be estimated by several different methods. Direct measurements of temperature have been made since 1850, and, prior to that, records can be deduced indirectly from, for example, ice cores formed over 100,000s of years.

One method uses this record together with energy-balance models and estimations of the effect of natural and anthropogenic processes to relate historical changes in carbon dioxide concentration to records of surface temperature change. Energy-balance models estimate the global average climate based solely on considerations of heat transfer (to the Earth from the Sun, and from the Earth via infrared radiation). These models make a number of assumptions, including how much heat is taken up by the oceans, and generally do not consider the geographical distribution of warming.

Another method to estimate equilibrium climate sensitivity uses computer simulations with complex global climate models. These models attempt to represent detailed physical processes, such as ocean heat uptake and climate feedbacks, and calculate a resulting sensitivity value. Each method is subject to its own approximations and uncertainties resulting in a range of estimates of sensitivity.

What was the basis for the statement in AR5?

Studies using different data sources and methodologies had produced a range of estimates of equilibrium climate sensitivity. In 2007 AR4 concluded that doubling of carbon dioxide concentration would lead to an equilibrium sensitivity in the range 2.0 to 4.5°C. In 2013, AR5 expanded the range to 1.5 to 4.5°C, to reflect some more recent studies based on past observations, but with no best estimate given. The range of transient climate response given in AR5 was 1.0 to 2.5°C.

 

Figure 2 – Global mean surface temperature projections (ICMP5, Coupled Model Intercomparison Project)

 

What do we know now?

Publications since AR5 continue to show equilibrium sensitivity estimates across the IPCC range. Those based on past observations and energy-balance models generally produce lower values than those derived from the more complex global climate models, including some suggesting ranges extending to values lower than those of AR5. There have, however, been advances in understanding of the reasons for this disparity.

One important advance is that it is now known that as the climate warms it becomes less effective at emitting heat to space, mainly as a result of regional variations in surface warming. This means that climate sensitivity derived from historical data (which typically fails to fully represent regional areas that may be warmer or cooler than the average) gives an underestimate of the value for high carbon dioxide atmospheres. It is also now clear that the very slow changes in patterns of ocean surface warming are inadequately represented in time varying global climate models resulting in an underestimate of climate sensitivity.

Insight has been evolving into the impact of localized processes on warming, for example volcanic eruptions or emission of industrial sulphate particles. The individual impact of these varies from type to type, but models ignoring such regional variations tend to give lower values for sensitivity. Another approach, in which global climate models that have been assessed on the basis of their ability to reproduce observed changes in cloud cover and properties, such as ice content and reflectivity, shows that the best performers generally have higher sensitivities.

Surface temperatures continue to be imperfectly observed. Gaps in the observation network and differences between measurement techniques for land and ocean mean that blending procedures are required to produce a global dataset. It has been demonstrated that incomplete geographical sampling of temperature can impact estimates of sensitivity. For example, the use of data with less coverage over the Arctic, where warming has been larger, has biased some climate sensitivity estimates to be too low.

How might this affect the IPCC statement?

Growing understanding of the complex, non-linear factors determining climate sensitivity is leading to improvements in methodologies for estimating it. A value below 2°C for the lower end of the likely range of equilibrium climate sensitivity now seems less plausible.

QUESTION TWO – How are methane concentrations changing and what does this mean for the climate?

Summary

After an apparent slow-down between 1999 and 2006, atmospheric methane concentrations have entered a period of sustained growth, increasing their contribution to surface warming.

In AR5, IPCC said:

Methane [concentrations] began increasing in 2007 after remaining nearly constant from 1999 to 2006

The exact drivers of this renewed growth are still debated.

What is this about?

Human activity results in a number of drivers of climate change. Carbon dioxide emissions have the largest overall effect, but, for example, increased concentrations of greenhouse gases such as methane and nitrous oxide add to carbon dioxide’s warming effect. The non- carbon dioxide drivers of climate change are a continuing research priority, in part because many influence local air quality as well as climate. Methane is the major greenhouse-gas driver of climate change after carbon dioxide, and there have been notable increases in its atmospheric concentration in recent years (and since AR5) that are not yet understood.

What was the basis for the statement in AR5?

Methane concentrations had increased markedly since the beginning of the industrial era, more than doubling from 770 parts per billion (ppb) approaching 1800 ppb in 2011. This increase was mostly attributed to human activity, including agriculture, waste, landfills, biomass burning and fossil fuel extraction. As for co2, evidence from air enclosed in polar ice cores demonstrated that present-day methane concentrations exceed any seen over the past 800,000 years. The growth rate of methane concentrations had not been steady; there was a slow-down in growth from 1990, which was particularly marked between 1999 and 2006.

At the time of writing of AR5, there was an indication that this period of slowdown had ended.

The total warming effect of methane emissions for the period 1750 – 2011 was assessed to be about 55% of the size of the warming effect of carbon dioxide emissions over the same period. This value includes methane’s direct warming effect and the impact of a number of indirect effects, notably the increase in ozone concentrations that results, via a sequence of atmospheric chemical reactions, from methane emissions.

 

Figure 3 – Global monthly mean methane (NOAA)

What do we know now?

The end of the slowdown in the growth of methane concentrations has been confirmed by continued global measurements. Annual-average concentrations increased from 1800 ppb in 2011, exceeded 1840 ppb in 2016 and may exceed 1850 ppb in 2017. Average growth rates now approach those seen in the 1980s prior to the slow-down.

Methane concentration is impacted by the rates of both emission and destruction, and the contributors to the recent changes remain debated. Evidence from the geographical distribution of changes, and from isotopic measurements, indicates that increased emissions have been strongest from biological sources, most likely associated with tropical agriculture and tropical wetlands, but increased emissions from fossil-fuels, due to their extraction and use, may also play a role.

There is little evidence of a significant increase in emissions from the Arctic. There is also further evidence that the rate of atmospheric destruction through chemical processes has slowed compared to what it was during the 1999 to 2006 period; the destruction rate is affected by human activity (including emissions of pollutants and concentrations of ozone), but the exact drivers of variations are not yet known.

How might this affect the IPCC statement?

There is no doubt that a period of renewed and sustained growth rate in methane concentrations has occurred since AR5. As a result, estimates of methane’s contribution to climate change have increased above those in AR5. Significant debate surrounds the factors that influence these trends, and projections of future emissions will need to focus on both emissions of methane and the rate at which chemical reactions destroy it.

QUESTION THREE – Was there a ‘pause’ in global warming?

Summary

In the 2000s the rate of surface warming was slower than in some previous decades, but the ocean continued to accumulate heat. Globally, 2015 and 2016 were the warmest years on record, and seen in this context the multi-decadal warming trend overwhelms shorter-term variability.

In AR5 IPCC said:

In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and interannual variability. Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.

What is this about?

Earth’s surface temperature, averaged globally over ocean and land areas, is one important measure of climate change. Since pre-industrial times, it has increased by around 1°C. However, the rate of increase has not been constant, and observational data assessed by the IPCC in AR5 suggested only a small increase between 1998 and 2012.

This period was referred to as a ‘hiatus’ or ‘pause’ in global warming, and raised questions in the media and elsewhere about whether it was evidence of problems with the models used to project future climate. Since then (and since AR5) global temperature has significantly increased.

What was the basis for the statement in AR5?

More than 90% of the heat energy associated with global warming accumulates in the ocean rather than in the atmosphere. Observations of ocean heat content and sea level rise suggested that over the period of slow surface temperature rise Earth’s climate system had continued to accumulate heat, particularly in the ocean beneath the surface.

It was understood that natural processes cause variability in surface temperatures from year-to-year and decade- to-decade, and hence in the rate of surface warming. Interactions within and between different parts of the climate system (known as ‘internal variability’), volcanic eruptions and fluctuations in the Sun’s energy output all contribute to the overall variability.

There were unresolved questions about the specific processes that had contributed to the slower surface warming seen between 1998 and 2012. The IPCC concluded that both internal variability and reduced heating of the Earth “due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle” were important factors. With regard to the comparison between models and observations, the IPCC again highlighted the importance of internal variability but acknowledged that weaknesses in some of the models and inaccurate estimates of some forcing agents (such as volcanic eruptions) might be an additional factor.

What do we know now?

Globally 2015 and 2016 were the warmest years in the surface temperature record, even allowing for the effects of the strong El Nino that affected both years. Seen in the context of the most recent years, the multi-decadal warming trend overwhelms shorter-term variability

The ‘pause’ apparent in the data used in AR5 can be attributed to two main factors: observational biases and the variability caused by natural processes. There is some evidence that changes in atmospheric aerosols (small particles in the atmosphere) caused by human activities may have been an additional factor.

Figure 4 – Global temperatures relative to 1850 – 1900 (Met/NASA/NOAA)

 

Improved understanding of observational biases has shown that the rate of surface warming between 1998 and 2012 was greater than the evidence available at the time of AR5 suggested.

There is now more evidence that the handling of observational gaps over the Arctic, a region of rapid warming, is important. When these biases are taken into account, a temporary slowdown in the rate of surface warming can still be seen in the data, albeit less prominently. Research since AR5 has strengthened the conclusion that this slowdown was primarily caused by natural variability, associated partly with variations in the surface temperatures of the Pacific Ocean.

The apparent differences in the rate of global surface temperature rise between models and observations have now been largely reconciled by taking proper account of internal variability, volcanic eruptions, and solar variability, in addition to the biases in the observational records. There are outstanding questions about the mechanisms that shaped the regional pattern of surface temperature change during the ‘pause’ – this is an area of ongoing research.

How might this affect the IPCC statement?

New evidence since AR5 supports the IPCC assessment that the period of slower surface warming that was observed between 1998 and 2012 was a short- term phenomenon not representative of long-term climate change. Despite the ‘pause’ in surface temperature rise, climate change carried on: the Earth continued to accumulate energy, particularly in the ocean, at a rate consistent with warming caused by human activities. In future the rate of surface warming is expected to continue to exhibit year-to-year and decade-to-decade variability in addition to the longer-term trend.

QUESTION FOUR – How high could sea level rise because of anthropogenic climate change?

Summary

Global mean sea level will likely rise by no more than a metre by 2100, but if warming is not limited, then its effects on the ocean and ice sheets could make a rise of several metres inevitable over centuries to millennia.

In AR5 IPCC said:

Global mean sea level rise for 2081 – 2100 relative to 1986 – 2005 will likely be in the ranges of 0.26 to 0.55 m for RCP2.6 … and 0.45 to 0.82 m for RCP8.5. Only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century.

What is this about?

The majority of large cities and 10% of the global population are located in low-lying coastal areas. Coastal floods are, generally, most likely to occur when storms drive the sea onto the land, but their increasing incidence during the 20th century was caused mainly by the rise in sea level (global mean of about 0.2 m since 1901), rather than greater storminess. Assessing the amount and rate of sea level rise into the future is therefore essential for assessing the risks and frequency of such flooding.

What was the basis for the statement in AR5?

Global mean sea level rise is caused by both expansion of the ocean as it gets warmer and addition of water to the ocean due to loss of ice from glaciers and the ice sheets of Greenland and Antarctica. During the 21st century, the largest projected contribution was from thermal expansion. However, the greatest uncertainty related to the contribution from ice sheets, which could become significantly greater after 2100. Surface temperature warming passing an estimated threshold in the range 2 to 4°C above pre-industrial temperatures could lead to the complete loss of the Greenland ice sheet over a millennium or more, with a 7 m rise in global mean sea level.

Warming of sea water which is in contact with those parts of the West Antarctic ice sheet resting on land below sea-level could cause partial disintegration of the ice sheet, through a process called ‘marine ice sheet instability’, and lead eventually to several additional metres of global mean sea level rise.

What do we know now?

Recent work has confirmed that observed warming of the ocean, contraction of glaciers and sea level change in the last few decades is due mainly to anthropogenic climate warming. An acceleration in the rate of sea level rise since the 1990s is consistent with increasing ice mass loss particularly from the Greenland Ice Sheet. There has recently been more attention paid to the West Antarctic Ice Sheet. Some glaciers there are currently retreating, and this has been suggested to be a sign that marine ice sheet instability is underway.

For 2100, under high emissions scenarios, most recently published estimates for the Antarctic contribution (mainly West Antarctica) to sea level rise do not exceed 0.4m. Global sea level rise from ice loss in both Greenland and Antarctica could however increase in rate beyond 2100, and will continue for centuries under all scenarios.

Concern about the likely long-term sea level rise is heightened by evidence that sea level was 6 – 9 m higher than today during the last interglacial period (125,000 years ago) when new climate reconstructions confirm that polar temperatures were comparable to those expected in 2100.

How might this affect the IPCC statement?

With the exception of one prominent study that projects the loss of most West Antarctic ice by 2500 under even moderate warming scenarios, other recent research is still broadly consistent with the AR5 assessment that marine ice sheet instability contribution to sea level rise will “not exceed several tenths of a meter” by 2100. Thus the AR5 projections still represent current understanding, although suggestions that the contribution could be greater than was previously assessed need further evaluation.

Quantitative uncertainties, reflected in the spread of results from recent studies, reinforce the need for better understanding of the processes leading to ice shelf and ice sheet retreat. It is moreover virtually certain that sea level rise will continue for many centuries. In a climate as warm as those projected in many models for 2100 and beyond under high emissions scenarios, large parts of both ice sheets would be lost over millennia, leaving sea level many metres higher than present.

QUESTION FIVE – Decreasing Arctic sea ice – is there any influence on the weather in middle latitudes?

Summary

The long-term decrease in Arctic sea ice extent continues and the effect of ice loss on weather at mid-latitudes has become a subject of active scientific research and debate.

In AR5 IPCC said:

The annual mean Arctic sea ice extent decreased over the period 1979 to 2012 with a rate that was very likely in the range 3.5 to 4.1% per decade (range of 0.45 to 0.51 million km2 per decade), and very likely in the range 9.4 to 13.6% per decade (range of 0.73 to 1.07 million km2 per decade) for the summer sea ice minimum (perennial sea ice).

What is this about?

The Arctic has warmed more rapidly than elsewhere. There are a number of reasons for this. Warming leads to a reduction in Arctic sea ice area, which leads to less of the Sun’s energy being reflected from the surface, and therefore additional warming during the summer, which is mainly absorbed by the ocean. During the winter the reduced Arctic sea ice area allows heat to escape from the ocean to the atmosphere above it.

Since 1979, when satellites first enabled a complete picture to be obtained, the reduction of sea ice is striking, particularly in the late summer minimum ice period, when the decrease is at a rate of more than 10% per decade.

Despite the long-term average increase in surface temperature at high-latitudes, there has been a wintertime cooling trend both in eastern North America and in central Eurasia over the last 25 years including a number of extremely cold winters (e.g. 2009/10 in northern Eurasia and 2014 in eastern North America). This period coincides with the period of pronounced Arctic sea ice decline. Some research has suggested that warming in regions of reduced sea ice leads to a weakening westerly polar jet stream that is more likely to meander. In such meanders very cold air may reach deep into middle latitudes.

What was the basis for the statement in AR5?

Increased levels of warming in the Arctic and the associated decrease in sea ice had been observed and were in general understood. However, at the time there was no indication of any particular link with changed patterns in extremes of mid-latitude weather and the lack of comment by IPCC reflected this.

What do we know now?

In the last five years, changes in the extent of Arctic sea ice has been consistent with a general decline and large natural variability from year to year. 2012 had a record September minimum, some 40% below typical values seen in the early 1980s. 2016 and 2017 have seen the smallest March maxima in sea ice area. There is no particular basis for making significant changes to the IPCC projections for future amounts of sea ice.

It is challenging to attribute observed changes in mid-latitude weather to Arctic sea ice loss, but there are indications from observations that sea ice loss may be causally linked to changes in wintertime atmospheric circulation over Eurasia that are consistent with the cooling seen there.

 

Figure 6 – Arctic sea ice area in September from 1979 to 2017 (National Snow and Ice Data Center)

 

There has been considerable use of computer models to investigate possible influences of Arctic warming on regional mid-latitude weather, and some theoretical, but conflicting, mechanisms have been proposed. If the weather systems stayed the same, enhanced Arctic warming would mean that the cold air blowing into middle latitudes from Arctic regions would be less cold.

However, there is some evidence from models that regional decreases in sea ice, such as in the Barents- Kara Sea (north of Finland and western Russia), can interact with the regional weather systems to increase the likelihood of very cold winter weather in Central Asia, as has been more prevalent since 1990. The nature and strength of linkages between Arctic sea ice loss and mid-latitude weather is a focus of considerable current research.

How might this affect the IPCC statement?

Arctic sea ice extent observed in the past five years is consistent with the statements made in AR5 on its general rate of reduction. It is likely that the next IPCC report will include more discussion on linkages between Arctic sea ice loss and midlatitude weather, particularly in Central Asia.

QUESTION SIX – Have temperature and rainfall extremes changed and how will they change in the future?

Summary

Climate change has increased the frequency of heatwaves. The effect on rainfall and tropical storms is more complex and harder to detect, but there is strengthening evidence that warming may increase the intensity of the strongest tropical storms.

In AR5 IPCC said:

It is now very likely that human influence has contributed to observed global scale changes in the frequency and intensity of daily temperature extremes since the mid-20th century, and likely that human influence has more than doubled the probability of occurrence of heat waves in some locations.

There are likely more land regions where the number of heavy precipitation events has increased than where it has decreased.

It is very likely that heat waves will occur with a higher frequency and duration.

Extreme precipitation events over most of the mid-latitude land masses and over wet tropical regions will very likely become more intense and more frequent by the end of this century, as global mean surface temperature increases.

What is this about?

Extreme events such as unusual heat, heavy rainfall, month- long droughts, or hourly very intense rainfall can have important impacts, for example on health, food production and infrastructure, especially if they happen infrequently which makes it difficult to adapt. As climate warms, some events that used to be rare, or even unprecedented in the context of today’s climate, will become more common, such as summer heat waves, while others will become less common, such as winter cold spells.

The warmer atmosphere increases the potential for heavy rainfall in general, even while some regions will receive less rainfall due to changes in atmospheric circulation. As temperature rises evaporation increases and will add to the potential for drought in some regions.

As well as these more direct effects, extreme events can also be affected indirectly by the impacts of changes in vegetation or ecosystems.

What was the basis for the statement in AR5?

The statements in AR5 were based on research considering observed trends in extremes on a globally widespread scale. Observed large-scale changes were compared with changes simulated over the 20th century in climate models, and with changes that are expected from natural climate variability only, attributing them to human influences. Confidence was higher for daily temperature extremes than rainfall extremes. There was also an emerging scientific literature determining to what extent climate change has influenced the likelihood of individual events, such as a particular observed heat wave event for example the European heatwave of 2003.

What do we know now?

Observations show that many extremes have continued to become more frequent and intense. Heat waves continued to increase in frequency even between 1998 and 2012, and research indicates an important interaction between dry conditions and heat waves.

Since AR5, analysis of specific extreme events has continued to indicate that human influences have made many individual heat waves much more likely, and cold spells less likely. Methods to quantify this change have improved, and different methods and approaches tend to lead to the same conclusions. Nevertheless some uncertainty remains as changes in atmospheric weather patterns can locally have a strong impact.

It is much more difficult to determine if humans have influenced other types of events, such as drought, or heavy rainfall events. Generally a warmer atmosphere is more conducive to heavy rainfall just because it can hold more water. However, natural climate variability in precipitation is very large, and changes in atmospheric circulation patterns have a substantial influence.

Therefore, results of attribution studies for precipitation- related events tend to depend on the type of event that is considered, and what assumptions are used. For example, results will often differ depending on whether a study considers how extreme the rainfall would have been without greenhouse gas increases for the exact same atmospheric conditions, or if it considers how extreme rainfall overall has changed in a region.

2017 was (at least until early October) a very active tropical cyclone season where severe damage was caused. IPCC AR5 indicated low confidence in observed long-term changes of intense tropical cyclone activity, and low confidence in the causes of those changes, but predicted more likely than not increases in intensity by the end of the century in the Western North Pacific and North Atlantic.

There is evidence from physical understanding and modelling that warming may increase the intensity of the strongest tropical cyclones. Also, analysis of model simulations and physical understanding suggest that heavy rainfall associated with tropical cyclones and other extreme storms should increase in a warmer atmosphere, all else being equal. Sea level rise exacerbates the impact of storm surges.

How might this affect the IPCC statement?

Further evidence supports the existing IPCC statements. Temperature extremes have become more frequent globally and rainfall extremes have increased in some regions and these trends are likely to continue in the future. More specific statements about the role that human influence has played in changing the frequency of specific types of events, particularly heat waves, are becoming possible.

Improved model simulations and physical understanding may strengthen confidence in projected changes in extreme daily and sub-daily rainfall, and in tropical cyclones and the heavy rainfall and the coastal inundation associated with them.

QUESTION SEVEN – Are there thresholds beyond which particularly dangerous or irreversible changes may occur?

Summary

There are a number of possible thresholds, but unless warming significantly exceeds expectations it is not expected that the most dangerous ones discussed here will be crossed this century.

In AR5 IPCC said:

It is unlikely that the AMOC [Atlantic Meridional Overturning Circulation] will collapse beyond the end of the 21st century for the scenarios considered but a collapse… for large sustained warming cannot be excluded.

It is very unlikely that methane from clathrates will undergo catastrophic release during the 21st century.

There is low confidence in projections of the collapse of large areas of tropical and/or boreal forests.

What is this about?

Several components of the Earth system might have thresholds or “tipping points”. If climate change passes certain levels, abrupt transitions could occur and parts of the climate system could be significantly altered. In some cases, these changes may be irreversible and in others it may take much longer to return to the original state even when the underlying drivers of climate change have ceased. Among the phenomena of concern are:

  • Collapse of the Atlantic Meridional Overturning Circulation, which transports ocean heat to North Atlantic surface waters, with widespread consequences for the climate.
  • Rapid release of methane from organic carbon
    in permafrost on land, or from methane hydrates (clathrates) below the ocean or causing significant further warming.
  • Large-scale dieback of the Amazon forest and consequential loss of ecosystem and carbon sink.

Potential thresholds for loss of large ice sheets leading to sea level rise, are discussed under the topic of sea level.

What was the basis for the statement in AR5?

AR5 concluded that collapse of the overturning circulation would cause significant global-scale climate disruption, including abrupt cooling around the North Atlantic. Weakening was expected in the 21st century, but an abrupt collapse was not, unless models seriously underestimate sensitivity to heat or freshwater, or the input of meltwater from Greenland is much faster than expected.

Warming at high latitudes will reduce the area of permafrost, and this will cause carbon dioxide and methane to be released to the atmosphere. However there was a wide range of estimates for the magnitude of these emissions. Ocean warming can destabilize clathrates below the sea floor, releasing methane to the ocean.

If large volumes reached the atmosphere, this would have a massive warming effect. However, AR5 concluded that oxidation would convert most of the methane to carbon dioxide before it reached the ocean surface, and the slow rate of heat penetration through the sediment meant that the destabilization of hydrates would be small on century scales.

AR5 recognized that the Amazon rainforest might have a critical threshold; particularly in relation to a rainfall volume below which large-scale dieback might be expected. However considering likely scenarios and the combined effects of carbon fertilization, warming, and changes in rainfall, fire and land use, they gave the cautious statement above.

What do we know now?

New palaeoclimatic measurements have strengthened the evidence linking changes in overturning circulation in the last glacial period to abrupt climate change, indicating that destabilization of overturning circulation can occur and is associated with climate disruption. However, these occurrences are not direct analogues for today’s interglacial period, because they were associated with inputs of meltwater from ice sheets much larger than the one that remains in Greenland.

Modern measurements confirm the variability of the Atlantic Meridional Overturning Circulation on daily, seasonal and interannual timescales, which makes detecting current trends challenging. Recent work suggests that climate models have biases favouring stability. This could imply that the likelihood of circulation collapse has been underestimated, but much more research is needed to reach firm conclusions.

Many new measurements have led to revised estimates of the amounts of carbon stored in permafrost, and the amounts of greenhouse gases released when permafrost thaws. These show that release of permafrost carbon will be a significant positive feedback for climate change; however, release is still expected to be prolonged and gradual rather than abrupt on decadal scales.

Several new measurements have suggested a limited influence of current clathrate releases (and indeed from permafrost on land) on the atmosphere. Assuming that the whole ocean does warm significantly, heat will reach larger volumes of clathrates, but this is expected to be gradual, implying a commitment to slow rather than catastrophic release to the ocean.

Many of the factors that influence the nature and health of forest ecosystems have been reported on, but recent modelling studies considering all the interactions and the ecosystem complexity show that there remains much uncertainty about the possibility of substantial spatially- coherent forest loss.

How might this affect the IPCC statement?

Based on current models, significant but gradual reductions in strength of the overturning circulation are expected if warming continues. However, sudden ocean circulation collapse remains unlikely, while still not being excluded, especially beyond 2100. Ocean warming implies a long-term commitment to some clathrate destabilisation with timescales up to millennia, but not necessarily to significant methane release into the atmosphere. The cautious IPCC statement about the Amazon as a whole is still valid.

In summary, gradual climate change could trigger abrupt changes – with large regional and potentially global impacts – associated with thresholds in the Earth system. The possibility of crossing any of these thresholds increases with each increment of warming. However, although surprises cannot be excluded, there is no compelling evidence that the thresholds discussed here will be crossed this century, or that the IPCC statements need significant amendment.

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To be continue: the final five questions of the Royal Society Climate Update and a further look at rising ocean acidity and its effect on mussels.