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August 11, 2021

The IPCC’s Urgent Warning from Climate Scientists


AR6 is a call to action: climate change impacts are pervasive, more pernicious than previously thought, and a present danger that demands bold, intelligent decisions.

by Dr. Pat Harr & Dr. Josh Hacker
Jupiter Intelligence

The overarching tone of the IPCC’s long-awaited 6th Assessment Report of August 9 is clear. Climate models are being validated, the impacts of extreme weather driven by climate change are occurring at a faster pace than previously projected, these impacts are global, and the likelihood of low-probability/high-impact events—such as the collapse of the Antarctic ice sheets—cannot be discounted.

AR6 is an unmistakable call to action from the scientific community to global decision-makers in the private and public arenas that—in addition to aggressively curtailing human-made causes of climate change—they must take immediate steps to develop and implement adaptation measures to make their societies, communities, economies, and infrastructure more resilient to climate-related disasters. These actions must have a foundation in information driven by forward-focused analysis of best-available-science General Circulation Models and climate data.

Nine Critical Insights From AR6

  1. A unique aspect of this report is its frank acknowledgment that, essentially, the goal of 1.5°C warming will be exceeded by mid-century under all climate scenarios. The report clearly calls for quick, effective, decisive, and global actions to limit warming into the second half of the 21st century. Additionally, it notes the necessity for equitable adaptation, such that all global impacts are addressed in an equitable and just manner.
  2. To support these informed adaptation strategies, the report highlights the need for physical climate information at global, regional, and local scales from multiple sources such as historical data, present observations, model simulations, and tailored diagnostic products.
  3. The report provides unambiguous evidence and unmistakably defines the implication of the nonstationary character of climate change, which must be a key element in effective climate data analytics for informed adaptation strategies.
  4. The report contains an exhaustive list of climate impacts to land, ocean, and atmosphere systems. Details in this list are abundant, but five key messages are:
  1. No portion of the globe will be spared climate change impacts.
  2. While there are large variations in the degree of confidence in the frequency and severity of impacts regionally, the levels of confidence are increasing.
  3. The key roles that oceans play in accounting for a vast majority of climate system heating, and the threat to ocean ecosystems and circulations (e.g. upper ocean stratification, ocean acidification, ocean deoxygenation) associated with ocean heating, accompanied by the warning that these conditions may persist for centuries or longer.
  4. Impacts from urban heat islands will continue to exacerbate periods of extreme heat.
  5. Sea level rise is “irreversible” and “will remain elevated for thousands of years.”
  1. Many of the impacts being observed today were predicted years and even decades earlier, but they were projected not to occur for decades to come. Model projections of impacts are being validated, but their timing is occurring much more rapidly than predicted.
  2. The report’s degree of confidence in attributing climate change to human actions is extremely high. It uses the phrase “unequivocal human influence” at the very beginning of the report, along with the phrase “highly likely” used repeatedly throughout the report.
  3. For the first time, the report reflects the increasing ability to define and understand paleoclimate reconstructions (the climate prevalent at a particular time in the geological past), observations of the current climate, and simulations of the future climate. These linkages and advances allow for high confidence comparisons of how the current climate and trends relate to paleo periods and how they will translate into future periods.
  4. Equilibrium climate sensitivity—defined as the change in temperature due to a doubling of carbon dioxide beyond pre-industrial levels—is a key metric. In this report, the equilibrium climate sensitivity has been raised to 3.0 degrees Celsius with a likely range of 2.5°C to 4.0°C. These values represent an increase in the magnitude of warming and a decrease in the variability—both significant factors that reflect increasing confidence in understanding past, present, and future climates. Importantly, while AR5 (issued in 2014) did not identify a “best estimate,” AR6 identifies it as 3.0°C.
  5. Compound events, which did not appear in AR5, have a new emphasis in AR6. While concurrent heat waves and droughts are a key example in AR6, the report stresses that compound extreme events that were considered to have a low likelihood in past and current climate will become more frequent. Additionally, the report expresses a high confidence that compound events can be expected to occur with increasing intensity plus increasing spatial and temporal scales.

Swift, Effective, and Global Action Required

This comprehensive assessment, seven years in the making, is a momentous achievement that will be scrutinized by scientists, governments, businesses, regulators, risk managers, and humanity at large for months to come.

However, it’s clear from the outset that AR6 shows a robust confidence that previously defined-as-acceptable limits to warming can no longer be met, and that—barring swift, effective, and global actions—future impacts of climate change will exceed limits thought highly unlikely a few short years ago.

Dr. Pat Harr is a Jupiter Science Fellow.

Dr. Josh Hacker is Jupiter’s Chief Scientist and a Co-founder.

Jupiter Intelligence is the global leader in climate analytics for resilience and risk management. For further information, please contact us at

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