Wednesday, 27 October, 2021


IPCC’s Sixth Assessment Report and Its Implications

Dr Kanan Purkayastha

IPCC’s Sixth Assessment Report and Its Implications
Dr Kanan Purkayastha

On 9 August 2021, the Intergovernmental Panel on Climate Change (IPCC) published the report of the Working Group 1 on the physical science basis of climate change, known as sixth assessment report (AR6WG1). The report incorporates new evidence from climate science. This essay is based on the IPCC’s 6th assessment report and author’s lecture delivered at the Royal Society of Chemistry on this subject.

Climate change is not a problem of the future. It is here and now. IPCC report mentioned that ‘it is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.’ The report has used the word ‘unequivocal’, an upgrade on the language of ‘clear’ used in the previous assessment report. In reporting the current state of the climate, IPCC has used new climate simulations, new analyses and methods combining multiple lines of evidence lead to improved understanding of human influence on a wider range of climate variables.

The report has informed that the observed increase in well-mixed greenhouse gas (GHG) concentrations since around 1750 is unequivocally caused by human activities. Since 2011, concentrations of GHG have continued to increase in the atmosphere, reaching annual averages of 410 parts per million (ppm) for carbon dioxide, 1866 parts per billion (ppb) for methane and 332 ppb for nitrous oxide in 2019. The report suggests that in 2019, atmospheric carbon dioxide were higher than at any time in at least 2 million years and concentrations of methane and nitrous oxide were higher than at any time in at least 800,000 years. In this calculation 1850-1900 is used as an approximation for pre-industrial conditions. While the level of GHG mentioned above is the concentration in air, land and ocean have also taken up globally about 56% per year of carbon dioxide emitted over the past six decades, with some regional differences.

It is interesting to note that some human drivers have exerted a cooling affect.  For example, GHGs contribution to surface temperature warming is 1.0 degree to 2.0 degree centigrade but other human drivers such as aerosols are contributed to a cooling by zero degree to 0.8 degree centigrade, other natural drivers changed global surface temperatures by -1 degree to 0.1 degree centigrade and internal variability change it by -0.2 degree to 0.2 degree centigrade. Taking these factors into account the likely range of human caused global surface temperature increase from 1850-1900 to 2010-2019 is 0.8 to 1.3 degree centigrade with a best estimate of 1.07 degree centigrade.

The report has identified that the globally averaged precipitation over land has likely increased since 1950 and human influence has contributed to the pattern of observed precipitation changes. Human influence is very likely the main driver of the global retreat of glaciers since the 1990s. Another phenomenon observed is the warming of the global upper ocean, which is up to 700m from the surface of the ocean. Carbon dioxide emissions are the main driver of current global acidification of the surface of the ocean. The report suggests that there is high confidence that oxygen levels have dropped in many upper ocean regions since the mid 20th century.

The global mean sea level increase is another alarming situation mentioned in the report. The average rate of sea level rise was 1.3 millimetre (mm) per year between 1901-1971, increasing to 1.9 mm per year between 1971-2006 and 3.7 mm per year between 2006-2018. The confidence level regarding this statistics is mentioned as ‘high’. The report highlighted the fact that human influence was very likely the main driver of sea level increase since at least 1971. There are two extremes also considered in the report: cold extremes and hot extremes. The report suggests that it is virtually certain that hot extremes have become more frequent and more intense across the region since 1950s, while cold extremes have become less frequent and less severe.

Radiative forcing is a concept used in all IPCC assessment reports. It is a concept used for quantitative comparisons of the strength of different human and natural agents in causing climate change. According to the report, human caused radiative forcing of 2.72 watt per square metre relative to 1750 has warmed the climate system. This warming is mainly due to GHG concentrations but partly reduced by cooling due to increased aerosol concentrations. Human caused net positive radiative forcing causes an accumulation of additional heating in the climate system. The observed average rate of heating of the climate system increased from 0.5 for the period of 1971-2006 to 0.79 watt per square metre for the period 2006-2018. The report also delineated the apportionment of heating system and also contributing factors for such heating.

Climate change is already affecting every inhabited region across the planet with human influence contributing many observed changes in weather and climate extremes. In case of South Asian region, that includes Bangladesh, observed change in hot extremes and heavy precipitation have increased and the statistical confidence regarding these matters are high. In case of observed change in agricultural and ecological drought in this region, there is a lack of evidence, so the report has mentioned it a ‘low agreement in the type of change’.

The report assesses the climate response to five emission scenarios that cover the range of possible future development of anthropogenic drivers of climate change. These set of scenarios drives climate model projections of changes in the climate systems. These projections account for solar activity and background forcing from volcanoes. The scenarios considered in the report are as follows: Very high and high GHG emission, where carbon dioxide emissions roughly double by 2050 and 2100 respectively; intermediate GHG emissions where carbon dioxide emissions remain around current levels until the middle of the century; low and very low emission scenarios where carbon dioxide emissions declining to net zero around or after 2050 and net negative emissions respectively.  Global surface temperature will continue to increase until at least the mid-century under all emission scenarios.

In a very high and high emission scenario, global surface temperature changes are expected to be 3.3-5.7 degree centigrade and 2.8-4.6 degree centigrade by 2100 respectively. In case of intermediate emission scenario, the temperature change is likely to be 2.1-3.5 degree centigrade by 2100 and for low and very low emission scenario the temperature change is likely to be 1.3-2.4 and 1.0 -1.8 degree centigrade respectively. The best estimate for very low emission scenario is 1.4 degree centigrade. The current level of carbon dioxide emission a year is 40 billion tonnes. For very low emission scenario that needs to fall to around 5 billion by 2050.  So, it is clearly our activities and choices that will determine where we end up over the next decades and centuries.

The report provides higher confidence levels for many statements made relevant to the physical science basis of the climate change. Only very low emission scenario can allow the temperature falls back to below 1.5 degree centigrade later this century. Therefore this would be the key task for countries that are meeting at the COP 26 climate summit in Glasgow, UK in November this year. Technologically, it is possible to suck carbon dioxide from the atmosphere and to see warming fall back to 1.4 degree centigrade by 2100, however political will, investment in mitigation and adaptation and behavioural change of human kind are important factors.

Above all, what has been delineated in the report is based on scientific understanding and key findings, which can be considered as a statement of facts. Some of the data presented in the report has been assessed on the basis of the levels of confidence such as very low, low, medium or intermediate, high and very high. These levels are based on the likelihood of outcome. This means that probability statistics has been used throughout the report. Philosopher Spinoza once wrote, ‘In practical life we are compelled to follow what is most probable; in speculative thought we are compelled to follow truth.’ So, at this point in time we should rely on IPCC’s sixth assessment report and follow what is most probable. Low-likelihood outcomes, such as ice sheet collapse, sudden ocean circulation changes, some compound effect of extreme events and warming that are substantially larger than the assessed likely range of future warming cannot be ruled out. So, we should develop some strategies for adaptation regarding any future eventualities.


The writer is a UK based academic,

scientist and author.