IPCC Sixth Assessment Report: Climate Change 2021 – Code Red for Humanity

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  • UN panel's (IPCC) historic climate change report has been termed as 'Code Red for humanity'. In this article, we will analyse the devastating impact of Human-induced Climate Change in every region of the planet – from the polar regions to the mountains, to the bottom of the ocean based on the evidentiary data provided in the report.


  • GS III- Conservation, Environmental Pollution & Degradation.
What is IPCC?
  • The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for the assessment of climate change. It is a key source of scientific information and technical guidance to the United Nations Framework Convention on Climate Change (UNFCCC), the Kyoto Protocol and Paris Agreement. The IPCC provides governments at all levels with the scientific information they can use to develop climate policies.
  • The IPCC is an organisation of governments that are members of the United Nations or the World Meteorological Organization (WMO). The IPCC currently has 195 members.

IPCC’s main activities are to prepare:

  • Comprehensive Assessment Reports on climate change, its causes, impacts and response options
  • Methodology Reports which provide practical guidance to Parties to help them prepare national greenhouse gas inventories
  • Special Reports on topics that inform the Assessment Reports
  • The IPCC does not undertake new research but synthesises published and peer-reviewed literature to develop a comprehensive assessment of scientific understanding, published in IPCC Assessment Reports.
  • The IPCC’s work is guided by a set of principles and procedures that govern all the main activities of the organisation. IPCC member governments and observer organisations nominate experts and the IPCC Bureau selects authors and editors, with expertise in a range of scientific, technical and socio-economic fields.
  • IPCC reports are the product of multiple drafting and review processes to promote an objective, comprehensive and transparent assessment of current knowledge
 Climate Change 2021: The Physical Science Basis Report
  • The overall assessment- the Sixth Assessment Report– is divided into three main parts. This report is the first part – on the physical science basis for climate change – and was delayed by almost a year due to COVID restrictions.
  • The next two parts will be released in 2022. One will cover the impacts, adaptation and vulnerability of, for example, people, ecosystems, agriculture, cities, and more. The other will cover the economics and mitigation of climate change.
  • The Sixth Assessment Report will culminate in a synthesis report, combining the first three parts, in September 2022.

Findings of the Report

We are set to pass 1.5C warming by 2040

  • The warming of recent decades has not been seen for millennia, is happening rapidly and almost everywhere on earth and has reversed a long-term global cooling trend. We need to go back around 125,000 years to find evidence of warmer global surface temperatures, spanning multiple centuries.
  • That leaves an increasingly narrow pathway to stabilising temperatures at 1.5C above pre-industrial levels by the end of the century, the most ambitious goal of the Paris Agreement.
  • Under all emissions scenarios outlined in the IPCC report, the earth’s surface warming is projected to reach 1.5C or 1.6C in the next two decades.
  • The threshold has come closer partly because scientists have incorporated new datasets in their estimate of historic temperature rise, including from the fast-warming Arctic. That adds 0.1C to the estimate of historic warming. High global emissions since the last assessment reports are continuing that trend.
  • For any chance of meeting the goal seen as essential to the survival of some vulnerable communities and ecosystems, drastic reductions in CO2 would be needed this decade and net zero emissions by 2050
  • With every additional increment of global warming, changes in extremes continue to become larger.
  • For example, every additional 0.5°C of global warming causes clearly discernible increases in the intensity and frequency of hot extremes, including heatwaves, and heavy precipitation, as well as agricultural and ecological droughts in some regions.
  • Discernible changes in intensity and frequency of meteorological droughts, with more regions showing increases than decreases, are seen in some regions for every additional 0.5°C of global warming. Increases in frequency and intensity of hydrological droughts become larger with increasing global warming in some regions.
  • 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. Projected percentage changes in frequency are higher for rarer events.

We are closer to irreversible tipping points 

  • The report sounds the alarm about the possibility of irreversible changes to the climate, often called tipping points.
  • For example, forests could start to die as temperatures rise, becoming less able to absorb carbon dioxide, leading to further warming. Or Antarctic ice sheets could become destabilised, leading to rapid sea-level rise.
  • “The probability of low-likelihood, high impact outcomes increases with higher global warming levels,” the report notes. “Abrupt responses and tipping points of the climate system, such as strongly increased Antarctic ice sheet melt and forest dieback, cannot be ruled out.” 
  • The melting of Antarctic ice sheets could cause sea levels to rise more than a metre by 2100 and 15 metres by 2500.
  • The “substantial increase in risks” was highlighted in a recent analysis which showed that parts of the Amazon are now emitting more carbon than they absorb.

Methane emissions are an important lever

  • For the first time, the IPCC has dedicated an entire chapter to “short-lived climate forcers” such as aerosols, particulate matter and methane. 
  • Methane levels are now higher than at any point in the past 800,000 years and are well above the safe limits outlined in AR5. Methane, which is released into the atmosphere from abandoned coal mines, farming and oil and gas operations, has a global warming impact 84 times higher than CO2 over a 20-year period. It is responsible for almost a quarter of global warming. 
  • Ecosystem responses to global warmings, such as thawing permafrost and wildfires, are highly likely to further increase concentrations of methane in the atmosphere. 
  • The authors state that a strong and rapid reduction in methane emissions would not only curb global warming but also improve air quality.
  • Despite its global warming impact, methane has received far less attention than CO2 and is not included in most countries’ climate pledges.
  • “A sharp reduction in methane would give you a short-term win, but it has largely been ignored by governments to date, all the focus has been on CO2 net-zero targets,” said Richard Black, senior associate at the Energy and Climate Intelligence Unit (ECIU).

Human Influence on Climate:

  • Decreases in global land monsoon precipitation from the 1950s to the 1980s are partly attributed to human-caused Northern Hemisphere aerosol emissions, but increases since then have resulted from rising GHG concentrations and decadal to multi-decadal internal variability.
  • Over South Asia, East Asia and West Africa increases in monsoon precipitation due to warming from GHG emissions were counteracted by decreases in monsoon precipitation due to cooling from human-caused aerosol emissions over the 20th century.
  • Increases in West African monsoon precipitation since the 1980s are partly due to the growing influence of GHGs and reductions in the cooling effect of human-caused aerosol emissions over Europe and North America.
  • It is likely that the global proportion of major (Category 3–5) tropical cyclone occurrence has increased over the last four decades, and 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.
  • There is low confidence in long-term (multi-decadal to centennial) trends in the frequency of all-category tropical cyclones. Event attribution studies and physical understanding indicate that human-induced climate change increases heavy precipitation associated with tropical cyclones Human influence has likely increased the chance of compound extreme events since the 1950s
  • This includes increases in the frequency of concurrent heatwaves and droughts on the global scale; fire weather in some regions of all inhabited continents; and compound flooding in some locations. 
  • Human-caused radiative forcing of 2.72 [1.96 to 3.48] W m–2 in 2019 relative to 1750 has warmed the climate system. This warming is mainly due to increased GHG concentrations, partly reduced by cooling due to increased aerosol concentrations. The radiative forcing has increased by 0.43 W m–2 (19%) relative to AR5, of which 0.34 W m–2 is due to the increase in GHG concentrations since 2011.
  • The heating of the climate system has caused global mean sea level rise through ice loss on land and thermal expansion from ocean warming. Thermal expansion explained 50% of sea-level rise during 1971– 2018, while ice loss from glaciers contributed 22%, ice sheets 20% and changes in land water storage 8%.
  • The rate of ice sheet loss increased by a factor of four between 1992–1999 and 2010–2019. Together, ice sheet and glacier mass loss were the dominant contributors to global mean sea level rise during 2006-2018. 
  • Cities intensify human-induced warming locally, and further urbanization together with more frequent hot extremes will increase the severity of heatwaves. Urbanization also increases mean and heavy precipitation over and/or downwind of cities and resulting in runoff intensity.
  • In coastal cities, the combination of more frequent extreme sea-level events (due to sea-level rise and storm surge) and extreme rainfall/river flow events will make flooding more probable.
  • Human influence has likely increased the chance of compound extreme events18 since the 1950s.
  • This includes increases in the frequency of concurrent heatwaves and droughts on the global scale; fire weather in some regions of all inhabited continents; and compound flooding in some locations. 

Carbon Cycle

  • The continued growth of atmospheric CO2 concentrations over the industrial era is unequivocally due to emissions from human activities. Ocean and land carbon sinks slow the rise of CO2 in the atmosphere.
  • Projections show that while land and ocean sinks absorb more CO2 under high emissions scenarios than low emissions scenarios, the fraction of emissions removed from the atmosphere by natural sinks decreases with higher concentrations.
  • Projected ocean and land sinks show similar responses for a given scenario, but the land sink has a much higher inter-annual variability and wider model spread. The slowed growth rates of the carbon sinks projected for the second half of this century are linked to strengthening carbon-climate feedbacks and stabilization of atmospheric CO2 under medium-to-no-mitigation and high31 mitigation scenarios respectively.
  • Land surface air temperatures have risen faster than the global surface temperature since the 1850s, and it is virtually certain that this differential warming will persist into the future. It is virtually certain that the frequency and intensity of hot extremes and the intensity and duration of heatwaves have increased since 31 1950 and will further increase in the future even if global warming is stabilized at 1.5°C.
  • The frequency and intensity of heavy precipitation events have increased over a majority of those land regions with good observational coverage (high confidence) and will extremely likely increase over most continents with additional global warming.
  • Over the past half-century, key aspects of the biosphere have changed in ways that are consistent with large-scale warming: climate zones have shifted poleward, and the growing season length in the Northern Hemisphere extratropics has increased (high confidence).
  • The amplitude of the seasonal cycle of atmospheric CO2 poleward of 45°N has increased since the 1960s, with increasing productivity of the land biosphere due to the increasing atmospheric CO2 concentration as the main driver (medium confidence). Global-scale vegetation greenness has increased since 1980.

Water Cycle

  • Human-caused climate change has driven detectable changes in the global water cycle since the mid-20th century, and it is projected to cause substantial further changes at both global and regional scales.
  • Global land precipitation has likely increased since 1950, with a faster increase since the 1980s. Atmospheric water vapour has increased throughout the troposphere since at least the 1980s (likely). Annual global land precipitation will increase over the 21st century as global surface temperature increases.
  • Human influence has been detected in amplified surface salinity and precipitation minus evaporation (P-E) patterns over the ocean. The severity of very wet and very dry events increase in a warming climate, but changes in atmospheric circulation patterns affect where and how often these extremes occur. Water cycle variability and related extremes are projected to increase faster than mean changes in most regions of the world and under all emission scenarios. Over the 21st century, the total land area subject to drought will increase and droughts will become more frequent and severe.
  • Near-term projected changes in precipitation are uncertain mainly because of internal variability, model uncertainty and uncertainty in forcings from natural and anthropogenic aerosols. Over the 21st century and beyond, abrupt human-caused changes to the water cycle cannot be excluded.

Impact on India

  • Glacial retreat in the Hindu Kush Himalayas; compounding effects of sea-level rise and intense tropical cyclones leading to flooding; an erratic monsoon; and intense heat stress are likely to impact India in recent years.
  • Most of these impacts are irreversible and hence cannot be remediated even if greenhouse gas emissions decline dramatically, the IPCC said.
  • The report said heatwaves and humid heat stress will be more intense and frequent during the 21st century over South Asia.
  • The Indian Ocean, which includes the Arabian Sea and Bay of Bengal, has warmed faster than the global average. The report indicates that sea surface temperature over the Indian ocean is likely to increase by 1 to 2 degrees C (°C) when there is 1.5°C to 2°C global warmings.
  • Over high mountains in Asia which include the Himalayas, snow cover has reduced since the early 21st century, and glaciers have thinned, retreated, and lost mass since the 1970s, the IPCC said, although the Karakoram glaciers haven’t recorded any major retreating trend.
  • Snow-covered areas and snow volumes will continue to decrease during the 21st century, snowline elevations will rise and glacier mass is likely to decline further as emissions rise. Rising global temperature and rain can increase the occurrence of glacial lake outburst floods (GLOFs) and landslides over moraine-dammed lakes, the IPCC warned.

  • India has recently suffered a spate of flooding and landslide disasters in the high mountains of Uttarakhand and Himachal Pradesh. A glacier breach on February 7 in Uttarakhand triggered flash floods in the Rishiganga and Dhauliganga valleys, sweeping away the Rishiganga hydel project and National Thermal Power Corporation’s Tapovan Vishnugad project. The disaster is feared to have killed over 200 people.
  • Heat extremes have increased while cold extremes have decreased, and these trends will continue over the coming decades over Asia. The Asia projection maps indicate annual mean temperatures will increase by 1-2°C relative to the 1850-1900 period in case of 1.5°C to 2°C global warmings. There are likely to be 90 to 120 days in a year with maximum temperatures above 35°C in case of 1.5 to 2°C warming and over 180 days in case of 4°C warmings.
  • Marine heatwaves will continue to increase. Fire weather seasons will lengthen and intensify, particularly in North Asia regions. Mean surface wind speeds have decreased and will continue to decrease in central and northern parts of Asia.
  • Glacier run-off in the Asian high mountains will increase up to the mid-21st century and subsequently, it may decrease due to the loss of glacier storage, the report said. Sea level around Asia in the North Indian Ocean has increased faster than the global average, with coastal area loss and shoreline retreat. Regional-mean sea level will continue to rise.

  • As far as monsoon is concerned, it has weakened in the second half of the 20th century mainly due to aerosols from human activity. Atmospheric aerosols are suspended liquid, solid, or mixed particles with highly variable chemical composition and size distribution. They are pollution particles that scatter solar radiation, and a few aerosol types can also absorb solar radiation thus having a cooling effect.
  • Though in the near term—next 20 years South and Southeast Asian monsoon and East Asian summer monsoon rain will be dominated by the effects of aerosols and internal variability, in the long-term, monsoon rain is likely to increase.
  • At 1.5°C global warming, heavy precipitation and associated flooding are projected to intensify and be more frequent in most regions in Africa and Asia, the report said adding that they have “high confidence” in the projection.
  • There is strengthened evidence since IPCC’s fifth assessment report released in 2014 that the global water cycle will continue to intensify as global temperatures rise, with rainfall and surface water flows projected to become more variable and unpredictable within seasons.
  • At the global scale, extreme daily rainfall events are projected to intensify by about 7% for each 1°C of global warming. Extreme rainfall events are defined as the daily precipitation amount over land that was exceeded on average once in a decade during the 1850–1900 reference period.
  • The proportion of intense tropical cyclones (categories 4-5) and peak wind speeds of the most intense tropical cyclones are projected to increase at the global scale with increasing global warming.
  • “A warmer climate will intensify very wet and very dry weather and climate events and seasons, with implications for flooding or drought, but the location and frequency of these events depend on projected changes in regional atmospheric circulation, including monsoons and mid-latitude storm tracks,” the report said.
  • Rainfall variability related to the El Niño–Southern Oscillation is projected to be amplified by the second half of the 21st century, it added.
  • Many regions are projected to experience an increase in the probability of compound events (i.e different types of extreme events happening at the same time or one after the other). This can be particularly alarming for India, experts pointed.
  • “2°C global warming has serious implications due to possible increases in extreme weather events like heatwaves, heavy precipitation, intensification of tropical cyclones etc.
  • Also, the variability of climate will increase suggesting more drought episodes where drying is happening and floods where wetting is happening. Dry becomes drier and wet becomes wetter. Another worrying fact is that we should expect compounding extreme weather events. For example, drought followed by heatwaves.

In Brief Highlights of IPCC report:

  • Climate change is intensifying the water cycle. This brings more intense rainfall and associated flooding, as well as more intense drought in many regions.
  • It is affecting rainfall patterns. In high latitudes, rainfall is likely to increase, while it is projected to decrease over large parts of the subtropics.
  • Changes to monsoon rain are expected, which will vary by region.
  • Coastal areas will see continued sea-level rise throughout the 21st century, contributing to more frequent and severe coastal flooding in low-lying areas and coastal erosion.
  • Extreme sea-level events that previously occurred once in 100 years could happen every year by the end of this century.
  • Further warming will amplify permafrost thawing, and the loss of seasonal snow cover, melting of glaciers and ice sheets.
  • Changes to the ocean, including warming, more frequent marine heatwaves, ocean acidification, and reduced oxygen levels have been clearly linked to human influence.
  • These changes affect both ocean ecosystems and the people that rely on them, and they will continue throughout at least the rest of this century.
  • For cities, some aspects of climate change may be amplified, including heat (since urban areas are usually warmer than their surroundings), flooding from heavy precipitation events and sea-level rise in coastal cities.

Way Forward suggested by IPCC to Combat Climate Change

  • Anthropogenic CO2 removal (CDR) has the potential to remove CO2 from the atmosphere and durably store it in reservoirs. CDR aims to compensate for residual emissions to reach net-zero CO2 or net-zero GHG emissions or, if implemented at a scale where anthropogenic removals exceed anthropogenic emissions, to lower surface temperature.
  • CDR methods can have potentially wide-ranging effects on biogeochemical cycles and climate, which can either weaken or strengthen the potential of these methods to remove CO2 and reduce warming, and can also influence water availability and quality, food production and biodiversity.
  • CDR leading to global net negative emissions would lower the atmospheric CO2 concentration and reverse surface ocean acidification (high confidence). Anthropogenic CO2 removals and emissions are partially compensated by CO2 release and uptake respectively, from or to land and ocean carbon pools.
  • CDR would lower atmospheric CO2 by an amount approximately equal to the increase from an anthropogenic emission of the same magnitude. The atmospheric CO2 decrease from anthropogenic CO2 removals could be up to 10% less than the atmospheric CO2 increase from an equal amount of CO2 emissions, depending on the total amount of CDR.
  • If global net negative CO2 emissions were to be achieved and be sustained, the global CO2-induced surface temperature increase would be gradually reversed but other climate changes would continue in their current direction for decades to millennia (high confidence). For instance, it would take several centuries to millennia for global mean sea level to reverse course even under large net negative CO2 emissions.
  • Achieving global net-zero CO2 emissions is a requirement for stabilizing CO2-induced global surface temperature increase, with anthropogenic CO2 emissions balanced by anthropogenic removals of CO2. This is different from achieving net-zero GHG emissions, where metric-weighted anthropogenic GHG emissions equal metric-weighted anthropogenic GHG removals.
  • For a given GHG emission pathway, the pathways of individual greenhouse gases determine the resulting climate response, whereas the choice of emissions metric used to calculate aggregated emissions and removals of different GHGs affects what point in time the aggregated greenhouse gases are calculated to be net-zero.
  • Emissions pathways that reach and sustain net-zero GHG emissions defined by the 100-year global warming potential are projected to result in a decline in surface temperature after an earlier peak.
  • Emissions reductions in 2020 associated with measures to reduce the spread of COVID-19 led to temporary but detectible effects on air pollution (high confidence), and an associated small, temporary increase in total radiative forcing, primarily due to reductions in cooling caused by aerosols arising from human activities (medium confidence).
  • Reductions in GHG emissions also lead to air quality improvements. However, in the near term, even in scenarios with a strong reduction of GHGs, as in the low and very low GHG emission scenarios, these improvements are not sufficient in many polluted regions to achieve air quality guidelines specified by the World Health Organization.
  • Scenarios with targeted reductions of air pollutant emissions lead to more rapid improvements in air quality within years compared to reductions in GHG emissions only, but from 2040, further improvements are projected in scenarios that combine efforts to reduce air pollutants as well as GHG emissions with the magnitude of the benefit varying between regions.


Some Important Terms from the Report

Global warming:

  • Global warming refers to the change of global surface temperature relative to a baseline depending upon the application. Specific global warming levels, such as 1.5°C, 2°C, 3°C or 4°C, are defined as changes in global surface temperature relative to the years 1850–1900 as the baseline (the earliest period of reliable observations with sufficient geographic coverage).
  • They are used to assess and communicate information about global and regional changes, link to scenarios and used as a common basis for WGII and WGIII assessments.


  • Emergence refers to the experience or appearance of novel conditions of a particular climate variable in a given region. This concept is often expressed as the ratio of the change in a climate variable relative to the amplitude of natural variations of that variable (often termed a ‘signal-to-noise’ ratio, with emergence occurring at a defined threshold of this ratio).
  • Emergence can be expressed in terms of a time or a 33 global warming level at which the novel conditions appear and can be estimated using observations or model simulations.

Cumulative carbon dioxide (CO2) emissions:

  • The total net amount of CO2 emitted into the atmosphere as a result of human activities. Given the nearly linear relationship between cumulative CO2 emissions and increases in global surface temperature, cumulative CO2 emissions are relevant for understanding how past and future CO2 emissions affect global surface temperature.
  • A related term – remaining carbon budget – is used to describe the total net amount of CO2 that could be released in the future by human activities while keeping global warming to a specific global warming level, such as 1.5°C, taking into account the warming contribution from non-CO2 forcers as well. The remaining carbon budget is expressed from a recent specified date, while the total carbon budget is expressed starting from the pre-industrial period.

Net-zero CO2 emissions:

  • A condition that occurs when the amount of CO2 emitted into the atmosphere by human activities equals the amount of CO2 removed from the atmosphere by human activities over a specified period of time. Net negative CO2 emissions occur when anthropogenic removals exceed anthropogenic emissions. 

Earth’s energy imbalance:

  • In a stable climate, the amount of energy that the Earth receives from the Sun is approximately in balance with the amount of energy that is lost to space in the form of reflected sunlight and thermal radiation. ‘Climate drivers’, such as an increase in greenhouse gases or aerosols, interfere with this balance, causing the system to either gain or lose energy.
  • The strength of a climate driver is quantified by its effective radiative forcing (ERF), measured in W m-2 7 . Positive ERF leads to warming and negative ERF leads to cooling. That warming or cooling in turn can change the energy imbalance through many positive (amplifying) or negative (dampening) climate feedbacks.


  • Attribution is the process of evaluating the relative contributions of multiple causal factors to a 13 observed change in climate variables (e.g., global surface temperature, global mean sea level change), or to 14 the occurrence of extreme weather or climate-related events.
  • Attributed causal factors include human activities (such as increases in greenhouse gas concentration and aerosols, or land-use change) or natural external drivers (solar and volcanic influences), and in some cases internal variability. 

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