A Response to the IPCC Report

10 August 2021

Anyone concerned about global catastrophic risks or existential threats to humanity should carefully note the findings of the Intergovernmental Panel on Climate Change (IPCC) in the first part of its sixth assessment report (AR6), released yesterday. Integrating findings from more than 14,000 peer-reviewed studies and outputs from the latest generation of global climate models, the report summarises what we know about the physical science basis for climate change. While the report has been comprehensively explained elsewhere, several key points bear highlighting.

First, a snapshot of affairs. It is now "unequivocal" that humans have warmed the planet, which has already, and increasingly, led to "widespread and rapid" changes to oceans, ice, and land, including more frequent and severe climate disasters like heatwaves, droughts, fires, floods and cyclones. Temperatures are the warmest they've been in roughly 125,000 years, at 1.09˚C above the pre-industrial baseline, and CO2 concentrations at their highest in at least 2 million years. Evidence for the causal link between global warming and extreme weather events has become much stronger since the IPCC's fifth assessment report (AR5), seven years ago. For many communities on the frontlines of climate impacts worldwide, these findings confirm an already-experienced reality. For the IPCC, it's merely confirming with a greater degree of confidence what previous reports had indicated was likely. There is clearly no place (if there ever was) for climate denial or delay.

The IPCC reaffirms that the more greenhouse gases (GHGs) are emitted, the warmer the planet gets and the more frequent and intense dangerous events become. The model projections use a new set of five plausible representative scenarios to explore a broad range of emissions trajectories that lead to possible futures of 1.4˚C to 4.4˚C warming by 2100 (at a "best estimate"), and projects the impacts on temperatures, precipitation patterns, and other crucial elements of the climate system for each of these. In other words, unless urgent action is taken, the record-breaking fires and floods, among other impacts, experienced across the world in recent weeks are only a taster of the coming decades.

Regional analysis of projected climate change

For those interested in how climate change will exacerbate and complicate other global hazards, including via the effects of hotter temperatures and changing rainfall on critical systems and infrastructure (a la Mani and Tzachor 2021), the IPCC's novel interactive portal with detailed regional information on past and projected climate change is a valuable resource. As the report states, even if global warming is limited to 1.5˚C, "some extreme events unprecedented in the observational record" will still increasingly occur. And no place on Earth is safe from these changes.

Irreversible changes and the longer term

Other parts of the report make it clear that a long-term, future-generations-oriented approach to governance has never been more important. For one thing, while global warming would cease if emissions stopped mid-century, other changes would continue. Sea level rise, ice sheet and glacier melt, permafrost thawing, and ocean acidification will continue and are "irreversible" on timescales of centuries to millennia. Even if global warming is limited to 1.5˚C this century, sea levels will rise by 2-3 metres during the next 2000 years under a best-case scenario, and 19-22 metres under a very high emissions scenario.

The report also considers how hot the planet could get in the longer term, under scenarios where emissions are not reduced to net zero this century. An intermediate emissions pathway would lead to 2.3-4.6˚C of warming by 2300, while a very high emissions pathway would cause 6.6-14.1˚C warming -- although the implications of such warming are not fully mapped out in the report.

Low-likelihood, high-impact risks and tipping points

Another particularly concerning theme of the report, discussed in greater detail than in previous IPCC work, is low-likelihood, high-impact risks. The report builds on improved understandings since AR5, highlighting that we cannot rule out "abrupt" changes due to passing tipping points in the Earth system, such as collapse of the Atlantic Meridional Overturning Circulation (AMOC), which would cause sudden shifts in weather patterns and the water cycle, including weakening the African and Asian monsoons. While AMOC will likely decline over the 21st century for all scenarios considered, the IPCC has only "medium confidence" that AMOC will not abruptly collapse before 2100. Nor is it possible to rule out large-scale Greenland ice sheet or West Antarctic ice sheet melt, abrupt permafrost thaw, or Amazon forest dieback. Such abrupt changes are possible even in a 1.5˚C future, and the risk increases with the level of warming.

The report also notes the low-likelihood, high-impact risk from "unpredictable and rare natural events not related to human influence on climate". Specifically, the IPCC mention "a sequence of large explosive volcanic eruptions within decades" causing substantial climate perturbations over several decades.


Not to be overlooked, either, are uncertainties about how much warmer the world will get in the future. One crucial uncertainty is climate sensitivity, which measures expected global warming if CO2 levels double compared to pre-industrial levels. AR6 narrows the "likely" range of climate sensitivity to 2.5-4˚C, with a central estimate of 3˚C, compared with the AR5 likely range of 1.5-4.5˚C. This is good, but substantial uncertainty still remains. Another, more troubling uncertainty comes from carbon-cycle feedbacks like methane release from permafrost thaw. This all means that levels of warming higher than the IPCC's "very likely" range for a given scenario cannot be ruled out.

Where to?

Where does this leave us? In one sense, the prescription is the same as ever. The scenarios make clear that rapid and drastic near-term emissions cuts are dearly needed. Looking at this another way, the IPCC brings some hopeful news. It reaffirms that a 1.5C future is still possible, if urgent action is taken. The world must cut CO2 emissions in half this decade, and reach "at least" net zero CO2 emissions and strongly reduce other GHG emissions by the middle of this century. Such a future would give us all the best chance of avoiding extremely dangerous low-probability, high-impact events, and would minimise the risk to critical systems worldwide. Governments must take urgent, radical and transformative action to phase out fossil fuels and reduce emissions to at least net zero, scaled according to responsibility and capability. Policymakers, politicians and corporate leaders must stop putting short-term interests before the current and future needs of the vast majority of people. Nothing less than the future of humanity is at stake.

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