杏吧原创

The world鈥檚 top climate scientists today released their first major review in eight years on the physical science of climate change, in a report approved by 195 countries. Here are 13 things we learned from the , about 3000 pages of which are simply a list of citations for the 14,000 scientific papers assessed.

1. The world has warmed by 0.1掳C more than previously thought

The amount the world has already warmed has been revised upwards compared with the last version of the IPCC鈥檚 report, released in 2013. This shift is due to improved observational records and a series of very warm years in the past decade. Earth鈥檚 global average surface temperature is now 1.09掳C above 1850-1900 levels. The rate of warming since the industrial revolution is unprecedented in at least 2000 years.

2. We have even greater certainty that our fossil fuel burning and activities are to blame

鈥淚t [the report] tells us that it is indisputable that human activities are causing climate change,鈥� says Hoesung Lee, chair of the IPCC. The report itself says: 鈥淚t is unequivocal that human influence has warmed the atmosphere, ocean and land.鈥� In 2013, the IPCC said human influence was only 鈥渃lear鈥�.

3. Earth is expected to reach or breach 1.5掳C of warming within two decades

All five emissions scenarios evaluated by the IPCC suggest we will miss the Paris Agreement鈥檚 toughest target, of holding warming to 1.5掳C, within the next 20 years. The central estimate is for the early 2030s. Even in the very lowest of scenarios for future greenhouse gas emissions, the threshold is 鈥渕ore likely than not鈥� to be hit.

4. Staying below 1.5掳C in the longer term is still possible

In the very lowest emissions scenario, temperatures are expected to fall back to 1.4掳C by 2100 after reaching 1.5掳C. 鈥淚f we rapidly reduce greenhouse gas emissions, if we can reach global net zero carbon dioxide emissions by around 2050, it is extremely likely we can keep global warming well below 2掳C,鈥� says Val茅rie Masson-Delmotte of the IPCC. That will require much stronger policies and measures than governments have in place now, though.

5. We have already locked in climate changes that will last thousands of years

Emissions to date mean some changes to the environment are now unavoidable, but we can slow them by cutting emissions faster. 鈥淒eep ocean warming, acidification and sea level rise are committed to ongoing change for millennia after global surface temperatures initially stabilize and are irreversible on human time scales,鈥� the report says.

6. Some air pollution cuts are likely to increase climate change

Some of the warming to date has been masked by the aerosols, or pollution, we have put into the air. The IPCC estimates a cooling effect of 0-0.8掳C so far. Cleaning up air pollution in the short term is good idea for human health and often also addresses some of the root causes of climate change. Nonetheless, doing so is 鈥渧ery likely鈥� to cause further warming in the next two decades, because pollution 鈥� aerosols 鈥� have a cooling effect on global warming.

7. Each fraction of warming and every extra tonne of CO₂ matters

Every additional 0.5掳C of global warming will cause 鈥渃learly discernible increases鈥� in the intensity and frequency of heatwaves, heavy rainfall, and droughts in some regions. That means we will still feel the benefits of efforts to reduce emissions even if we breach 1.5掳C or 2掳C. Passing such thresholds doesn鈥檛 mean we should give up on net zero.

8. The pandemic has had little impact on climate change

The covid-19 pandemic led to a huge drop in global emissions last year but it hasn鈥檛 changed the big picture on climate change. The impact of reduced emissions caused by lockdowns and economic slowdowns were 鈥渦ndetectable鈥� beyond natural variability due to their temporary nature, said the IPCC.

9. CO₂ removals are going to be important

Using technology and tree-planting to remove CO₂ from the atmosphere does work, and is a plausible way of stabilising global temperature rises. The report concludes that CO₂ removal can work, and that it would actually reverse warming, says Joeri Rogelj at Imperial College London, an IPCC author. 鈥淪o that is good news.鈥� However, each tonne of CO₂ removed could be 10 per cent less effective at cooling than each tonne of CO₂ emitted is at warming 鈥� in other words, we can鈥檛 simply take CO₂ out of the atmosphere and pretend we never emitted it.

10. We cannot rely on nature as an excuse for business as usual

Forests, oceans and other 鈥渃arbon sinks鈥� will get less efficient at absorbing CO₂ if we continue with high emissions. 鈥淲hile natural land and ocean carbon sinks are projected to take up, in absolute terms, a progressively larger amount of CO₂ under higher compared to lower CO₂ emissions scenarios, they become less effective, that is, the proportion of emissions taken up by land and ocean decrease with increasing cumulative CO₂ emissions,鈥� says the report.

11. Geoengineering the planet is probably a bad idea

One proposed solution to climate change, putting particles in the atmosphere to reduce the amount of the sun鈥檚 energy reaching Earth, is probably a bad idea. The report says the approach, and other ones under a suite of 鈥渟olar radiation modification鈥� options, could offset global warming but is likely to cause abrupt changes to water cycles, could have regional climate impacts and poses a risk because suddenly stopping would cause 鈥渞apid climate change.鈥� Amanda Maycock at the University of Leeds, UK, and an IPCC author, says such strategies 鈥渉ave a huge number of risks associated with them鈥�.

12. There is no longer any doubt about the links between climate change and extreme weather

Heatwaves, floods and other events have all been blamed on climate change in recent years. In 2013, the IPCC said it had 鈥渄etected changes in some climate extremes鈥� due to human influences such as burning fossil fuels. Today the IPCC said 鈥渋t is now an established fact鈥� that the increased frequency and intensity of such events is due to humanity鈥檚 emissions.

13. The Arctic is likely be ice-free at points in coming decades

Even in a net zero world where emissions follow the lowest of the IPCC鈥檚 five scenarios, the Arctic is likely to fall to a 鈥減ractically ice-free state鈥� (meaning there are less than a million square kilometres of ice) at least once before 2050. Under the highest emission scenarios, that ice-free state will become the norm for late summer by the end of the century.

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