A new Assessment of the World’s Climate is out. The News Isn’t Good

In 2014, the UN Intergovernmental Panel on Climate Change (IPCC) released its Fifth Assessment Report (AR5). As with previous reports, AR5 contained the latest findings of Climate Change experts from all relevant disciplines, as well as projections about the near future. In short, the AR5 and its predecessors were assessments of the impact anthropogenic Climate Change was having on the planet and how we could avoid worst-case scenarios.

On Aug. 9th, 2021, the IPCC released a report titled Climate Change 2021: the Physical Science Basis. Combining the latest advances in climate science and multiple lines of evidence, this first report paints a rather bleak picture of the remainder of the 21st century. At the same time, it presents a call to action and shows how mitigation strategies and reducing greenhouse gas emissions will ensure a better future for all.

The Working Group I report is the first installment of the IPCC’s Sixth Assessment Report (AR6), which will be completed and released to the public by 2022. As with previous reports, it provides a summary of global warming trends and assesses the likely impact by region. But this time around, the report focuses more heavily on recommendations for curbing carbon dioxide (CO2) and other greenhouse gases.

Given the predictions made in this latest report, especially where positive feedback mechanisms are involved, this should come as no surprise. “This report reflects extraordinary efforts under exceptional circumstances,” said Hoesung Lee, Chair of the IPCC, in a press release that accompanied the release of the AR6. “The innovations in this report, and advances in climate science that it reflects, provide an invaluable input into climate negotiations and decision-making.”

A total of 234 authors from 66 countries contributed to the creation of the Working Group I Report (31 coordinating authors, 167 lead authors, and 36 review editors) and 517 contributing authors. Originally scheduled for release in April of 2021, the report was delayed for several months by the COVID-19 pandemic, making the AR6 the only report that was the subject of a virtual approval session. As IPCC Working Group I Co-Chair Valérie Masson-Delmotte stated:

“It has been clear for decades that the Earth’s climate is changing, and the role of human influence on the climate system is undisputed… This report is a reality check. We now have a much clearer picture of the past, present and future climate, which is essential for understanding where we are headed, what can be done, and how we can prepare.”

Warming Trends

Consistent with the previous Assessment Reports, the AR6 is based on improved observational datasets that assess historical warming and progress in the scientific community to how Earth’s climate responds to anthropogenic greenhouse gas emissions. Also, like its predecessors, the AR6 establishes that an average global increase of 1.5°C (2.7°F) between now and 2100 is the best-case scenario.

Meanwhile, it once again establishes that an increase of 2°C (3.6°F) is the scenario that is to be avoided. This is not the worst-case, mind you, as any scenario comes down to the greenhouse gas (GHG) emissions that are produced by human activity (or as a consequence of it) every year. While this may not sound like major changes, it’s important to understand that the average represents all variations, depending on the region, season, and even day and night cycles.

Under a 1.5°C increase, the hottest days will be up to 3°C (5.4°F) around Earth’s mid-latitudes. At higher latitudes, the coldest nights will be 4.5°C (8.1°F) warmer; while in the Arctic, temperatures will become warmer by 5.5°C (9.9°F) and cold spells will be shorter. In even the “best case” scenario, the resulting impact will be considerable and will range from increased wildfires and drought to severe flooding and sea-level rise (all of which are already being witnessed).

To begin, the report explains how average global temperatures have already increased by approximately 1.1°C (~2°F) since 1850-1900, which is directly attributable to human activity and rising greenhouse gas emissions. The report then provides new estimates of the chances of crossing the 1.5°C threshold in the coming decades concludes that unless there are immediate, rapid, and large-scale reductions in greenhouse gas emissions, this goal will be beyond reach.

In fact, it estimates that even an average increase of 2°C will be unavoidable, which will entail more severe ecological consequences. In this scenario, temperatures nearer to the equator will increase by up to 4ºC (7.2ºF) while the higher altitudes and the Arctic will experience warming of up to 6°C (10.8°F) and 8°C (14.4°F), respectively. The positive feedbacks this creates, where Arctic ice sheets and permafrost are depleted and trigger the release of large pockets of methane, will also be more significant.

To condense the findings, the AR6 indicates that an average increase of 1.5°C will result in increased heat waves, longer warm seasons, and shorter cold seasons. Meanwhile, an average increase of 2°C will mean that extremes in heat will more often reach critical tolerance thresholds for agriculture and health. In short, the former scenario is not pretty, but at least it’s sustainable. In the latter and any other more dire scenarios, life will become untenable in certain parts of the planet.

Impact by Region

The AR6 also provides a detailed breakdown of the effects of this warming by region. For example, further warming trends will continue to intensify the planet’s water cycle, which means more intense drought and heatwaves in many regions and more intense rainfall and associated flooding in others. Rainfall patterns will also continue to be affected, with increased precipitation in the higher latitudes and decreased precipitation (especially where monsoons are involved) over large parts of the tropics.

As expected, coastal areas will continue to see an average rise in sea levels throughout the 21st century under both scenarios. This will mean more frequent “nuisance flooding,” where storms cause coastal waters to reach inland on a regular basis, causing property damage and drainage systems to overflow. It will also mean that more extreme sea-level events that occurred once a century are likely to happen annually.

The increased loss of ice sheets and permafrost and less seasonal snow cover will increase how much solar energy the polar tundra and Arctic sea absorb. This will trigger the release of methane deposits in both areas, a “super greenhouse gas,” which will compound the problem further. Increases in ocean temperatures, marine heatwaves, ocean acidification, and reduced oxygen levels will also have a dire impact on ocean ecosystems and fisheries.

Another point made by the report is how these effects will differ for people living in urban areas and rural areas. Throughout this century, a major demographic shift is expected where most of the Earth’s population will come to live in major cities. The effects of climate change will be amplified here in several ways, as urban areas are generally warmer than their surroundings and coastal cities are vulnerable to flooding and sea-level rise.


Luckily, the report was not all doom and gloom. Beyond presenting the likely impacts that Climate Change will have in the coming century, it also shows how strong and sustained reductions in emissions of greenhouse gases would limit Climate Change. While the benefits for air quality (and related public health concerns) would be felt quickly, it would take 20 to 30 years before global temperatures began to stabilize.

In addition, the AR6 provides (for the first time) a detailed overview of regional impacts and information that can inform risk assessment, adaptation, and other decision-making in the coming years. It also offers a new framework that helps users understand what physical changes in the climate – increased heatwaves, drought, wildfires, rainfall, flooding, cold spells, etc. – could mean for society and ecosystems.

Lastly, the new report also reflects major advances in our understanding of the role climate change plays when it comes to intensifying specific weather and climate events – what is known as the “science of attribution.” The report also emphasizes how human agency is a double-edged sword, where our actions have the potential to alter the climate in ways that are positive (as well as negative).

The effects Climate Change will have regionally can be explored in detail using a new tool developed by the IPCC, known as the Interactive Atlas. The Summary for Policymakers, a Technical Summary (TS), FAQs sheet, Media Essentials, and the full AR6 report can all be accessed on the IPCC AR6 website. As IPCC Working Group I Co-Chair Panmao Zhai summarized:

“Climate change is already affecting every region on Earth, in multiple ways. The changes we experience will increase with additional warming. Stabilizing the climate will require strong, rapid, and sustained reductions in greenhouse gas emissions, and reaching net zero CO2 emissions. Limiting other greenhouse gases and air pollutants, especially methane, could have benefits both for health and the climate.”

Preventing the types of scenarios that are predicted by 2050 and the end of the 21st century will be nothing short of herculean. At this juncture, it’s not just a matter of curbing our emissions substantially; it will also likely call for large-scale carbon sequestration and perhaps some ecological engineering as well. It’s a big mountain to climb, but it’s significantly more pleasant (and more cost-effective) than the alternative!

Further Reading: IPCC