Last year, the IPCC unveiled their Sixth Assessment Report: The Physical Science Basis. A total of 234 scientists from 66 countries are behind the first chapter of the report, which draws on 14,000 research papers to come up with conclusions on how the Earth’s climate is changing as a result of human activity.

Whilst the messages of this report remain the same as previous IPCC reports, the urgency is greater. In a period of heatwaves, wildfires, and flooding, and in the midst of preparations for COP26, this report could come at no better time.

The IPCC report is not short on words, but I’ve compiled the key takeaways from this important publication for those short on time. For those in the same ‘climate nerd club’ as myself, the full report can be downloaded here.

1. The human influence on the climate crisis is crystal clear.

The summary of the report begins with a clear opening message: “It is unequivocal that human influence has warmed the atmosphere, ocean, and land.” This warming has occurred at a rate that is unprecedented in at least the last 2000 years.

Each of the past four decades has been successively warmer than its predecessor. The global surface temperature was 1.1°C higher in 2011–2020, compared to 1850–1900, with 1.07°C of this rise being linked to human-caused factors. The report further outlines the human influence on land precipitation, upper-ocean warming, rising sea levels, decline in Arctic sea ice, and glacial retreat.

2. The scale of changes across the climate system is unprecedented.

The scale of change across many areas of the climate system is alarming, with the report providing key data across a number of climatic aspects:

In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years.
The global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years.
In 2011–2020, the annual average Arctic sea ice area reached its lowest level since at least 1850.
Global mean sea level has risen faster since 1900 than over any preceding century in at least the last 3000 years.

3. Extreme climate events have become more frequent and severe.

Climate change is already affecting every inhabited region across the globe. Since the prior report (AR5), evidence of human influence on observed extreme climate changes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, has strengthened.

Data highlights the certainty of human-induced climate change being the main driver of more frequent and intense hot extremes and heavy precipitation across most land regions since the 1950s, with the hot extremes observed in the past decade being extremely unlikely to have occurred without the human influence on the climate system.

Human-induced climate change has contributed to increases in agricultural and ecological drought and has increased the chance of ‘compound extreme events’ since the 1950s (including concurrent heatwaves and droughts at the global scale, fire weather, and compound flooding).

4. Global warming of 1.5°C and 2°C are seemingly within reach.

The report sets out 5 ‘future scenarios’ that explore projections of climate system changes in response to a range of GHG emissions. The analysis demonstrates that under the ‘immediate, high and very high’ emissions scenarios, global warming of 1.5°C and 2°C would be exceeded.

Under the ‘low’ emissions scenario, it is likely that the 1.5°C warming target would be exceeded, though warming could remain under 2°C (best estimate = 1.8°C, range = 1.3–2.4°C). Under the ‘very low’ emissions scenario, there is greater optimism. This scenario demonstrates the potential for global surface temperature to decline back to below 1.5°C warming towards the end of this century, with only a temporary overshoot witnessed.

Remaining at today's emissions levels would result in a global surface temperature rise of 2°C by mid-century.

5. Additional warming only makes things worse!

With every additional increment of warming, projected changes in climate extremes are larger in their frequency and intensity. At the global scale, with every 1°C of warming “extreme daily precipitation events will intensify by around 7%”.

Since the previous report, strengthened evidence demonstrates that continued global warming will further intensify the global water cycle, including its variability, global monsoon precipitation, and the severity of wet and dry events, with implications on flooding and drought.

Furthermore, under scenarios with increasing CO2 emissions, the proportion of emissions ‘taken in’ by natural land and ocean sinks is smaller, making our carbon sinks essentially ‘less effective’.

A ‘stand-out’ in this IPCC report is the more regional approach, with its provision of physical climate information across the global, regional, and local scales. Whilst all regions are projected to experience the impact of global warming, some impacts vary in their intensity and frequency by region.

6. There’s no going back from some changes in the climate system.

Many climate changes are irreversible for centuries to millennia, especially those relating to the ocean, ice sheets, and global sea level. Past GHG emissions have committed the ocean to future warming over the rest of the century, and changes are irreversible at the centennial to millennial time scales in relation to global ocean temperature, deep ocean acidification, and deoxygenation.

Mountain and polar glaciers will continue to melt for decades or centuries and, at centennial timescales, loss of permafrost carbon is irreversible following permafrost thaw.

Sea level rise is also committed for centuries to millennia, with projections showing a rise of 2-3m if warming is limited to 1.5°C, 2-6m if limited to 2°C, and 19-22m with 5°C of warming over the next 2000 years.

7. The climate we experience in the future depends on our decisions now.

Since the prior report, estimates of remaining carbon budgets have been improved. The reaching of net-zero anthropogenic CO2 emissions at a global scale is required to stabilize global warming. Though, it’s not just CO2 emissions that should remain focal. Strong, rapid, and sustained reductions in methane emissions would also limit warming and would additionally improve air quality.

Due to the global lockdowns and restrictions imposed by the COVID-19 pandemic, emissions reductions witnessed in the prior year led to a temporary (though still detectible) impact on air pollution and radiative forcing. However, climate responses were undetectable above natural variability, and atmospheric concentrations of CO2 still rose in 2020.

Whilst climate tipping points can’t be ruled out, urgent action to address global emissions would minimize the likelihood of these tipping points occurring. Global decisions now should look to rapidly reduce emissions to the ‘very low’ or ‘low’ emissions scenarios in order to have any chance of keeping within the 2°C global surface temperature warming. With COP27 around the corner, there is the chance for nations to make real, impactful, and sustained decisions to alter the course of this planet before it truly becomes too late.

Beth

The Ipcc's Sixth Assessment Report in 5 Minutes