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The Changing Climate: Part 3 (2021 IPCC Report)

In Part 3 we are going to look at the most recent Assessment Report No.6 (AR6) published by the Intergovernmental Panel on Climate Change.  This document is 3,946 pages – so, we will focus only on the executive summary for policy makers.  This summary circles on the key points of the report.  It was published August 7, 2021.

The United Nations


The Intergovernmental Panel on Climate Change

Who is the IPCC?

It is a panel developed by the United Nations to address the topic of global climate change.  It was created in 1988 by the World Meteorological Organization and the United Nations Environmental Programme.  It currently has 195 members and contributions from thousands of scientists.  The panel scientists volunteer their time to review all of the scientific papers that are used in the assessment reports.  The panel meets one or more times a year1.


IPCC Assessment Report Six (AR6) – August 7, 2021

All of the assessment reports are based on scientific information and presented as statements of fact, or statements at a level of confidence using an IPCC calibrated language (such as likely, medium confidence, high confidence, etc.).  The summary for policy makers is divided into four sections.  We will summarize each one.

  1. Current State of the Climate
  2. Possible Climate Futures
  3. Climate Information for Risk Assessment and Regional Adaptation
  4. Limiting Future Climate Change


  1. Current State of the Climate


A.1) 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 definition of unequivocal is “leaving no doubt, unquestionable, clear”.  Using that term here indicates the panel is convinced that the current warming of our planet is caused by human activity.  Thus, there is something we can do to turn it around.


Other statements in this section include:

  • Each of the last four decades has been successively warmer than any decade preceded it since 1850.
  • Globally averaged precipitation over land has likely increased since 1950, with a faster rate of increase since the 1980s.

This term likely is one of the terms used by their calibrated language model.  Suggesting the confidence level is not as high – but likely.

  • It is likely that human influence contributed to the pattern of observed precipitation change since the mid-20th century, and extremely likely that human influence contributed to the pattern of observed changes in near-surface ocean salinity.
  • Mid-latitude storm tracks have likely shifted poleward in both hemispheres since the 1980s.
  • Human influence is very likely the main driver of the global retreat of glaciers since the 1990s and the decrease in the Arctic Sea ice area between 1979-1988 and 2010-2019 is about 40% in September and 10% in March.
  • It is virtually certain that the global upper ocean (0-2400 feet) has warmed since the 1970s and extremely likely that human influence is the main driver.
  • Global mean sea level increased by 0.7 feet between 1901 and 2018.
  • Changes in land ecosystems since 1970 are consistent with global warming. Climate zones have shifted poleward in both hemispheres and growing seasons have, on average, lengthened by up to two days per decade since the 1950s.  This is stated with high confidence.

A.2) 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 thousand years. 


  • In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years (high confidence), and concentrations of CH4 (methane) and N2O (nitrous oxide) were higher than at any time in at least 800,000 years (very high confidence).
  • Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years (high confidence).
  • In 2011-2020, annual average Arctic Sea ice area reached its lowest level since at least 1850 (high confidence).
  • Global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3000 years (high confidence).

A.3) Human-Induced climate change is already affecting many weather and climate extremes in every region across the globe.  Evidence in observed changes in extremes such as heatwaves, heavy precipitation, droughts, tropical cyclone, and (in particular) their attribution to human influence, has strengthen since AR5.


  • It is virtually certain that hot extremes have become more frequent and more intense across most land regions since the 1950s, while cold extremes have become less frequent and less severe, and that human-induced climate change is the main driver (high confidence).
  • The frequency and intensity of heavy precipitation events have increased since the 1950s over most land area for which observational data are sufficient for trend analysis (high confidence), and human-induced climate change is the main driver (likely).
  • 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 the cyclones reach their peak intensity has shifted northward. These changes cannot be explained by internal variability alone (medium confidence).  There is low confidence in long-term (multi-decade 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 (high confidence), but data limitations inhibit clear detection of past trends on the global scale.
  • Human influence has likely increased the chance of compound extreme events since the 1950s.



  1. Possible Climate Futures

B1. 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 reduction in CO2 and other greenhouse gas emissions occur in the coming decades.

  • Compared to 1850-1900, global surface temperature averaged over 2081-2100 is very likely to be higher than 1.0°C to 1.8°C under the VERY LOW greenhouse gas emissions scenario. 1°C – 3.5°C in the INTERMEDIATE scenario.  And 3.3°C to 5.7°C under the VERY HIGH scenario.
  • Global surface temperature in any single year can vary above or below the long-term human-induced trend, due to substantial natural variability.

B2. Many changes in the climate system become larger in direct relation to increasing global warming.  They include increases in the frequency and intensity of hot extremes, marine heatwaves, heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic Sea ice, snow cover, and permafrost.

  • It is virtually certain that the land surface will continue to warm more than the ocean surface. It is virtually certain that the Arctic will continue to warm more than the global surface temperature.
  • With every additional increment of global warming, changes in extremes continue to become larger.
  • Some mid-latitude and semi-arid regions, and the South American Monsoon region, are projected to see the highest increase in the temperature of the hottest days, about 1.5 to 2 times the global rate of warming (high confidence).
  • It is very likely that heavy precipitation events will intensify and become more frequent in most regions with additional global warming.
  • Additional warming is projected to further amplify permafrost thawing, and loss of seasonal snow cover, of land ice, and of Arctic Sea ice (high confidence).

B3. Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation and severity of wet and dry events.

  • There is strengthened evidence since AR5 that the global water cycle will continue to intensify as global temperatures rise (high confidence), with precipitation and surface water flows projected to become more variable over most land regions within seasons (high confidence) and from year to year (medium confidence).
  • A warmer climate will intensify very wet and very dry weather and climate events and seasons, with implications for flooding and droughts (high confidence), but the location and frequency will depend on projected changes in regional and atmospheric circulation, including monsoons and mid-latitude storm tracks.

B4. Under scenarios with increasing CO2 emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere.  

  • Based on model projections, under the intermediate scenario that stabilizes atmospheric CO2 concentrations this century, the rates of CO2 taken up by the land and oceans are projected to decrease in the second half of the 21st century (high confidence).

B5. 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.

  • Past greenhouse gas emissions since 1750 have committed the global ocean to future warming (high confidence).
  • Mountain and polar glaciers are committed to continue melting for decades or centuries (very high confidence).
  • It is virtually certain that global mean sea level will continue to rise over the 21st
  • In the longer term, sea level is committed to rise for centuries to millennia due to deep ocean warming and ice sheet melt and will remain elevated for thousands of years (high confidence).


  1. Climate Information for Risk Assessment and Regional Adaptation

C1. Natural drivers and internal variability will modulate human-induced changes, especially at regional scales and in the near term, with little effect on centennial global warming.  The modulations are important to consider in planning for the full range of possible changes.


What this is basically stating is that the predictions made with the computer models, can be altered by natural drivers (such as nearby ocean currents, forested land, or even urban-induced weather) as well as internal variability (such as the humidity at time of cyclone formation, wind conditions during a heavy rainfall, etc.).  These are regional impacts and are not accounted for in the models.


C2. With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climate impact-drivers.  Changes in several climatic impact-drivers would be more widespread to 2°C compared to 1.5°C global warming and even more widespread and/or pronounced for higher warming levels.


Basically… as the model predictions occur, their impacts on the climate can change enough that the data inputted into the model could be warmer than anticipated and thus, increase the outcome predicted.

  • It is very likely to virtually certain that regional mean relative sea level rise will continue throughout the 21st century, except in a few regions with substantial geologic land uplift rates.
  • Cities intensify human-induced warming locally, and further urbanization together with more

frequent hot extremes will increase the severity of heatwaves (very high confidence).

  • Many regions are projected to experience an increase in the probability of compound events with higher global warming (high confidence).


C3. Low-likelihood outcomes, such as ice sheet collapse, abrupt ocean circulations changes, some compound extreme events and warming substantially larger than the assessed very likely ranges of future warming cannot be ruled out and are part of risk assessment.


Unpredictable and rare natural events not related to human influence on climate may lead to low likelihood, high impact outcomes.


  1. Limiting Future Climate Change

D1. From a physical science perspective, limiting human-induced global warming to a specific level requires limiting CO2 emissions, reaching at least net zero CO2 emissions, along with strong reductions in other greenhouse gases.  Strong, rapid and sustained reductions in CH4 (methane) emissions would also limit the warming effect resulting from the declining aerosol pollution and would improve air quality.

  • Removing CO2 by humans leading to net negative emissions would reverse surface ocean acidification (high confidence).
  • If global net negative CO2 emissions were to be achieved and be sustained, the global CO­2-induced surface temperature increase would gradually reverse, but other climate change 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 (high confidence).

D2. Scenarios with very low or low greenhouse gas emissions lead within years to discernible effects on greenhouse gas and aerosol concentrations, and air quality, relative to high and very greenhouse gas emissions scenarios.  Under these contrasting scenarios, discernible differences in trends of global surface temperature would begin to emerge from natural variability within around 20 years, and over longer time periods for many other climate impact-drivers (high confidence). 


Basically… at high confidence – if we can reduce the amount of greenhouse gas emissions to match the low emission scenario used by the models, the climate outcomes would be reduced as well.

  • 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).
  • Scenarios with very low or low GHG emissions would have rapid and sustained effects to limit human-caused climate change, compared with scenarios with high or very high GHG emissions, but early responses of the climate system can be masked by natural variability.




  1. Based on the report – there is no doubt that human-induced greenhouse gases have warmed the planet.
  2. The climatic changes observed over the last century are unprecedented.
  3. Weather and climate extremes are occurring.
  4. No matter what we do, global surface temperatures will continue to increase until mid-century.
  5. If the annual increases in greenhouse gas emissions continues, their weather/climate impacts will increase as well.
  6. If emissions continue at their present rate, the carbon sinks (methods of removing carbon – both land and ocean) will become less effective.
  7. With high confidence – some impacts are irreversible – oceans will continue to warm, ice sheets and glaciers will continue to melt, and sea level will continue to rise for centuries and millennia.
  8. There are local and regional features (nearby ocean currents, forested – or deforested areas, nearby mountain ranges) that can alter the impacts of the computer models for better or worse, than the scenarios presented.
  9. Some regional climate impacts can affect the outcomes as such as multiple events can occur.
  10. By reducing greenhouse gases, we can alter some of the impacts predicted in these scenarios.




Much of this sounds bad… and it is.  As we mentioned in Part 1 “everyone is talking about the climate… but no one is doing anything about it” is not 100% true… many are doing something about it.  But based on this AR6 report, not enough.  The AR6 report also mentions that many of the climate/weather scenarios can be altered if we do begin to make some behavioral changes on the amount of greenhouse gasses we emit AND look into sources of carbon sequestering.


In Part 4 of this series, we will look closer at the impacts this is having on the Florida panhandle.

Photo: Molly O’Connor



1 The Intergovernmental Panel on Climate Change


2 IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis.

Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.