There will be no ifs and buts or maybes any more. Sceptics can now quit the debate on climate change. Eight years after the last report (AR5), Part 1 of the Sixth Assessment Report (AR6) of the United Nations Intergovernmental Panel on Climate Change (IPCC) has delivered the most definitive verdict on global warming and its impacts through the 21st century and beyond to the extent that the present understanding of the science of climate has allowed the community of world climate scientists—which has produced this mammoth report—to do.
While this part of the report is the contribution of the Working Group I (WG I) of the IPCC that looks at the Physical Science Basis of Climate Change, the remaining two parts, which would be the contributions of WG II (on Climate Change Impacts, Vulnerability and Adaptation) and WG III (on Mitigation), and the final Synthesis Report are expected to be released by September 2022. The authors of WG I built on 14,000 scientific papers, and the report itself—which has been approved by 195 governments—runs into about 4,000 pages.
The authors have also issued a summary for policymakers (SPM), which highlights the important and significant findings of the assessment by WG I. This article is essentially based on the SPM. The report uses the following phrases throughout to indicate the likelihood of a finding or conclusion, which are used in the article as well (in italics) to mean the same: virtually certain 99-100 per cent probability, extremely likely 95-100 per cent, very likely 90-100 per cent, likely 66-100 per cent, more likely than not 50-100 per cent, about as likely as not 33-66 per cent, unlikely 0-33 per cent, very unlikely 0-10 per cent, extremely unlikely 0-5 per cent, and exceptionally unlikely 0–1 per cent.
By bringing to bear “multiple lines of evidence” and a fresh analytical approach to model simulations, rather than relying solely on simulations (as was the case with AR5), the report in the main says: “It is unequivocal that human influence has warmed the atmosphere, ocean and land.” This, it adds, has brought about “widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere”.
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As for projections about climate future, the report issues a dire warning. It says: “Global surface temperature will continue to increase until at least the mid-century under all emissions scenarios considered. Global warming of 1.5 °C and 2 °C will be exceeded during the 21st century unless deep reductions in CO 2 and other greenhouse gas (GHG) emissions occur in the coming decades.”
These observations in themselves are not new to non-sceptics, but the improvements in model simulations, observationally based estimates, historical data from palaeoclimate archives and methodological advances have increased the authors’ level of confidence in their conclusions and projections by significantly reducing windows of uncertainty.
Human activity and climate extremes
In recent times, particularly in the last couple of years, we have witnessed increased frequency of many weather and climate extremes in every region of the world. It is often asked if these are a result of climate change. The report says: “Evidence of observed changes in extremes, such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since AR5…. Human influence has likely increased the chance of compound extreme events since the 1950s.”
The recent numbers are a telling testimony to how human activities (anthropogenic) have continued to play havoc with the weather and climate. Since 2011 (measurements used in AR5), well-mixed GHG concentrations have continued to increase in the atmosphere, reaching annual averages of 410 parts per million (ppm) for carbon dioxide (CO 2 ), 1,866 parts per billion (ppb) for methane (CH 4 ), and 332 ppb for nitrous oxide (N 2 O) in 2019. In 2019, atmospheric CO 2 concentrations were higher than at any time in at least two million years, and concentrations of CH 4 and N 2 O were higher than at any time in at least 800,000 years, the report states with high confidence and very high confidence respectively.
The report also says that the scale of recent changes “across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years” (Fig. 1). Also, as we will see later, with increasing CO 2 emissions, the report projects with high confidence that land and ocean carbon sinks, which have taken up over half of anthropogenic CO 2 emissions in the past, will be less effective at slowing the accumulation of CO 2 in the atmosphere.
The global surface temperature increase owing to human activities from 1850-1900 to 2010-2019 is estimated to be 1.07 °C. The warming has increased faster since 1970 than in any other 50-year period over at least the last 2,000 years, the report states with high confidence . Fig. 2 shows the relative contributions of various drivers to this increase. The report says: “It is very likely that well-mixed GHGs were the main driver of tropospheric warming since 1979 (partly reduced by cooling due to aerosol concentrations), and extremely likely that anthropogenic stratospheric ozone depletion was the main driver of cooling of the lower stratosphere between 1979 and the mid-1990s.”
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Recall that the (binding) treaty arrived at by 196 nations at the Paris Conference of Parties (COP 21) in December 2015 (which came into force by November 2016) aims to limit global warming to well below 2 °C, preferably to 1.5 °C, compared with the pre-industrial levels (1850-1900) century. To achieve this goal, countries have committed to attain peaking of GHG emissions as quickly as possible so as to make the world climate carbon neutral by mid-century.
But countries are nowhere on course to meet their Paris commitments. Each of the last four decades, according to the report, has been successively warmer than any decade that preceded it since 1850. Global surface temperature in the first two decades of the 21st century (2001-2020) was 0.99 °C higher than during 1850-1900, and the increase during 2011-2020 has been 0.2 °C higher (at 1.09 °C) than during 2001-2010. In fact, temperatures during this decade exceeded those of the most recent multi-century warm period, around 6,500 years ago relative to 1850-1900, according to the report. Temperature increase over land (1.59 °C) has been higher than over the ocean (0.88 °C), which is not surprising since water reflects much of the solar radiation and also has a higher heat capacity.
With relatively less (medium)confidence , the report said that the globally averaged precipitation over land has likely increased since 1950, with a faster rate of increase since the 1980s, and human activities contributed to the pattern of observed precipitation changes. However, it is able to say with greater confidence that human influence contributed to the pattern of observed changes in near-surface ocean salinity.
Anthropogenic factors are again very likely the main drivers of the global retreat of glaciers since the 1990s and the decrease in Arctic sea ice area between 1979-1988 and 2010-2019 (about 40 per cent in September and about 10 per cent in March), the report says. With medium confidence , the report adds that the nature of glacier retreat, with almost all of the world’s glaciers retreating synchronously since the 1950s, is unprecedented in at least the last 2,000 years.
According to the report, that human activities also very likely contributed to the melting of the Greenland Ice Sheet over the past two decades. With high confidence , it says that in 2011-2020, the annual average Arctic sea ice area reached its lowest level since at least 1850. In particular, the late summer Arctic sea ice area was smaller than at any time in at least the past 1,000 years. However, there was no significant trend in Antarctic sea ice area from 1979 to 2020 because of regionally opposing trends and large internal variability. The report also says that there is only limited evidence for anthropogenic influence on the Antarctic Ice Sheet mass loss.
The report states with high confidence that the global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3,000 years. The chief reasons for this were ice loss on land and thermal expansion from ocean warming owing to heating of the climate system. While thermal expansion explained 50 per cent of the sea level rise during 1971-2018, ice loss from glaciers contributed 22 per cent, ice sheets 20 per cent and changes in land water storage 8 per cent. According to the report, the rate of ice sheet loss increased by a factor of four between 1992-1999 and 2010-2019. The combined effect of glacier mass and ice sheet loss was the dominant contributor to global mean sea level rise during 2006-2018.
Assessing weather extremes, the report says: “It is virtually certain that hot extremes (including heatwaves) have become more frequent and more intense across most land regions since the 1950s, while cold extremes (including cold waves) have become less frequent and less severe.” It concludes with high confidence that anthropogenic climate change was the main driver of these changes and some hot extremes observed over the past decade would have been extremely unlikely otherwise.
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While land monsoon precipitation decreased globally from the 1950s to the 1980s, due in part due to anthropogenic aerosol emissions (which cause cooling) in the Northern Hemisphere, the reversal since then has been a result of rising GHG concentrations and decadal to multi-decadal internal variability, the report says, but only with a medium level of confidence. However, it states with high confidence that increases in monsoon precipitation over South Asia, East Asia and West Africa owing to warming from GHG emissions were counteracted by decreases in monsoon precipitation caused by anthropogenic aerosol cooling over the 20th century.
On tropical cyclones, the report says that it is likely that the fraction of major tropical cyclones (Category 3-5) has increased globally over the last four decades. Further, the latitude where tropical cyclones in the western North Pacific reach their peak intensity has shifted northward. These changes cannot be explained by internal variability alone, the report says with medium confidence . At present, the assessment is not good enough to infer with any significant degree of confidence the long-term (multi-decadal to centennial) trends in the frequency of tropical cyclones of all categories.
An important measure of how the climate responds to radiative forcing (RF) is what is called the equilibrium climate sensitivity (ECS). (RF is the net of energy flux in the atmosphere caused by natural and/or anthropogenic factors; positive RF causes warming. ECS is the long-term global surface temperature rise with the doubling of atmospheric CO 2 concentration.)
According to the report, relative to 1750, RF caused by human activities was 2.72 watts/m 2 in 2019 and there was a 19 per cent increase over the AR5 value (2011) of 0.34 W/m 2 . With improved knowledge of climate processes and multiple lines of evidence, AR6 has arrived at the best value of 3 °C for the ECS and the likely range, estimated with high confidence , is 2.5 °C-4 °C, as compared to 1.5 °C-4.5 °C in AR5, which did not yield a best estimate. This has been an important development in climate science that has greatly helped in the above assessments of climate change effects of warming.
Having assessed the current state of climate and unequivocally determined that certain unbridled human activities have been the cause of the immense damage already done to the climate system, the question is what kind of climatic future lies ahead for the earth and its nearly 8 billion inhabitants. That naturally depends on the pathway that the people of the world choose to adopt during the remaining decades of the 21st century.
New emission scenarios
To answer this question, the report has considered five new emission scenarios that account for a broader range of anthropogenic climate change drivers—GHG emissions, land use and air pollution—as compared to AR5 and has made model projections for the 21st century. These begin in 2015 and include scenarios with high and very high GHG emissions and CO 2 emissions that are double from current levels respectively by 2100 and 2050 (called SSP3-7.0 and SSP5-8.5 respectively); scenarios with intermediate GHG emissions and CO 2 levels remaining at current levels until 2050 (called SSP2-4.5); and scenarios with very low and low GHG emissions and CO 2 emissions declining to “net zero” around or after 2050, following by varying levels of net negative CO 2 emissions (called SSP1-1.9 and SSP1-2.6 respectively).
(Net zero emissions are achieved when anthropogenic emissions of GHGs into the atmosphere are balanced by anthropogenic removals over a specified period. When referring only to CO 2 emissions, it is termed as net zero CO 2 emissions.)
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For the assessment, the report has used climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) of the World Climate Research Programme, which are reported to have a wider range of climate sensitivity than the CMIP5 models used in AR5, largely owing to better incorporation of cloud feedback in them. Combined with the approach of multiple lines of evidence, AR6 has assessed the very likely range for the various projections and has been able to provide the best estimate for them as compared to AR5. Apparently, it is for the first time that an IPCC report has assessed future changes in surface temperature, ocean warming and sea level by combining multi-model projections with observational constraints arising from simulations of past warming as well as the AR6 assessment of climate sensitivity.
According to the projections of the report, compared with 1850-1900, global surface temperature averaged over 2081-2100 is very likely to be higher by 1.0 °C to 1.8 °C under the very low GHG emissions scenario considered, and by 2.1 °C to 3.5 °C in the intermediate scenario and by 3.3 °C to 5.7 °C under the very high GHG emissions scenario. (See table for the likelihood of exceeding the temperature guardrails of 1.5 °C and 2.0 °C increase relative to 1850-1900 for different GHG emission scenarios.)
Global warming
The many changes in the climate system that we have witnessed so far will only become enhanced with increasing global warming even if the world manages to stay within the guardrail limits of 1.5 °C and 2.0 °C increase in global surface temperature by the end of the century. According to the report, with every additional 0.5 °C of global warming, increases in the intensity and frequency of hot extremes, including heatwaves, and heavy precipitation, as well as all types of droughts—meteorological, hydrological, agricultural and ecological—in some regions will become very perceptible.
It is also virtually certain that the Arctic will continue to warm more than global surface temperature, at more than two times the rate of global warming, leading to large reductions in Arctic sea ice, snow cover and permafrost. As regards decrease of Antarctic sea ice, the report says that projections could be made only with low confidence .
On the global scale, the report projects with high confidence that extreme daily precipitation events will intensify by about 7 per cent for each 1 °C of global warming. It also projects with high confidence that the proportion of intense tropical cyclones (categories 4-5) and peak wind speeds of the most intense tropical cyclones will increase globally. These should be of particular concern for a country like India, which is already witnessing intensification of monsoon rain and cyclones. “There will be an increasing occurrence of some extreme events unprecedented in the observational record with additional global warming, even at 1.5 °C of global warming,” the report predicts.
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There is now greater evidence that the global water cycle will continue to intensify with increasing global temperatures, when precipitation and surface water flows will become more variable over most land regions within seasons and intra-annually, says the report with higher confidence than at the time of AR5. As the warming increases, very wet and very dry weather and climate events and seasons will intensify, with implications for flooding or drought, the report said with high confidence .
Pointers for India
However, it adds that the location and frequency of these events will depend on regional atmospheric circulation, including monsoons and mid-latitude storm tracks. Of particular relevance for India in this context is the report’s observation (with high confidence ) that although monsoon precipitation is projected to increase over South Asia with temperature rise, rainfall variability related to the El Nino–Southern Oscillation (ENSO) will get amplified by the second half of the 21st century in all the scenarios except for low and very low GHG emission ones (SSP1-1.9 and SSP1-2.6).
The capacities of the earth’s natural sinks of land and ocean to absorb anthropogenic CO 2 emissions, while being huge, are not infinite. As global CO 2 emissions continue to grow, these capacities will also gradually reduce, making absolute reduction in emissions a much more effective approach to combat effects of global warming than the concept of ‘net zero’ or carbon neutrality—even in the near term—as is being now widely advocated.
However, as the report points out with high confidence , in absolute terms these natural sinks are projected to take up a progressively larger amount of CO 2 under higher compared to lower CO 2 emissions scenarios, but become less effective; that is, the proportion of emissions taken up by land and ocean decrease with increasing cumulative CO 2 emissions. This will result in a higher fraction of emitted CO 2 remaining in the atmosphere.
The report also projects with high confidence that in many regions there will be an increase in the probability of compound events and concurrent extremes with higher global warming. In particular, concurrent heatwaves and droughts are likely to become more frequent. These aspects could make issues of region-specific risk assessment and adaptation that much more complex and difficult as we move towards the end of the century unless absolute and drastic emission reductions are undertaken globally before 2050.
The report warned: “Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level.” According to the report, over the rest of the 21st century, the ocean is likely to warm from 2-4 to 4-8 times the 1971-2018 change depending on the future emission scenarios. On the basis of multiple lines of evidence, the report projects with virtual certainty that upper ocean stratification, ocean acidification and ocean deoxygenation will continue to increase in the 21st century at rates dependent on future emissions.
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Similarly, it is virtually certain that the global mean sea level will continue to rise over the 21st century. (Fig. 3 shows the relative sea level increases relative to 1995-2014 under different emission scenarios.)
The report says: “Global mean sea level rise above the likely range—approaching 2 m by 2100 and 5 m by 2150 under a very high GHG emissions scenario—cannot be ruled out due to deep uncertainty in ice sheet processes.”
Every tonne of CO 2 /GHG emitted will add to global warming in the near future. There is a near linear relationship between cumulative CO emissions (measured in gigatonnes of CO 2 , or GtCO 2 ) and global warming (Fig. 4). So, from the perspective of limiting future climate change, depending on the specific limit of temperature increase that we choose to pursue, the available space in the atmosphere for emissions (or remaining carbon budget), after accounting for historical and current emissions, is correspondingly limited.
The report’s best estimate is that for each 1,000 GtCO 2 of cumulative CO 2 emissions the likely increase in global surface temperature is 0.45 °C. It says: “From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO 2 emissions, reaching at least net zero CO 2 emissions, along with strong reductions in other GHG emissions.”
Over the 1850-2019 period, a total of about 2,390 GtCO 2 of anthropogenic CO 2 was emitted.
According to the report, since AR5, a new methodology led to improved estimates of remaining carbon budgets by integrating results from multiple lines of evidence. The significant improvement in the estimates has enabled the authors to ascertain when climate responses to emissions reductions will become discernible above natural climate variability and internal variability.
According to the report, compared with high and very high GHG emissions scenarios, in those with very low or low GHG emissions discernible effects on GHG and aerosol concentrations, and air quality, begin to emerge from natural variability within around 20 years.
Emphasis on net zero approach
In an otherwise timely report to remind the world where it is headed in climatic terms, one would possibly disagree with it on its overemphasis on the net zero approach to achieve carbon stabilisation in the atmosphere as against absolute and deep cuts in the current emissions, particularly by the developed countries, which are predominantly responsible for the shrinking carbon budget because of their large historical emissions since 1750 when the industrial revolution began.
The report said: “Achieving global net zero CO 2 emissions is a requirement for stabilising CO 2 -induced global surface temperature increase, with anthropogenic CO 2 emissions balanced by anthropogenic removals of CO 2 .”
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In principle, yes; anthropogenic carbon dioxide removal (CDR) leading to global net negative emissions would lower the atmospheric CO 2 concentration and reverse surface ocean acidification, as the report projects with high confidence . Since CDR would lower atmospheric CO 2 by an amount approximately equal to the increase from an anthropogenic emission of the same magnitude, the report argues in favour of the net zero approach for the near term.
However, the net zero approach, as some climate scientists have recently pointed out, has turned out to be a red herring (see “Climate scientists: concept of net zero is a dangerous trap”, The Conversation (online), April 22, 2021) because no CDR technology (for example, carbon sequestration techniques) has matured enough to become operational and offsetting tree/forest plantations has never been commensurate with the emissions, not to mention the various uncertainties involved in the different CDR approaches.
The report would have done well to evaluate the different CDR approaches on their relative efficacies. It would have, in fact, done better to lay stress on the imperative of absolute emissions reduction rather than allowing a free run on emissions hoping for CDR methods to adequately compensate for them, which would only contribute to making the guardrails of temperature rise unattainable.
