Science now admits irreversible effects of climate change on the planet

Climate change, "undoubtedly" caused by human activity, has taken the world into its warmest period in 2,000 years, and will have irreversible effects for millennia, warns a new report by the Intergovernmental Panel on Climate Change (IPCC).

The IPCC document, which has been analysing the effects of climate change on the planet for the United Nations since 1988, indicates for example that mountain and polar glaciers will continue to melt for decades or even centuries, even if emissions are reduced.

"The report is a reality check. We now have a much clearer picture of past, present and future climate, which is essential to understand where we are going, what we can do, and how we should prepare," Valérie Mason-Delmotte, co-chair of the report's panel of experts, said at the launch of the report.

The report also predicts irreversible changes on the scale of thousands of years in temperature, acidification and deoxygenation of the oceans.

It also predicts that sea levels will continue to rise irreversibly, by 28 to 55 centimetres by the end of the century compared to current levels even with net-zero emissions.

"For decades the IPCC has warned us of the dangers of global warming, the world listened, but did not act forcefully enough, and as a result the problem is here and no one is safe," said Inger Andersen, executive director of the UN Environment Programme.

The report says humans have played an "undeniable" role in warming the atmosphere, ocean and land, leading the world to a rise in temperatures unparalleled in the last 2,000 years.

The study, which takes advantage of improvements in palaeoclimate research, shows that the current temperature rise is comparable to what is now considered the warmest period in the last 100,000 years, which occurred 6,500 years ago (the so-called Holocene climatic maximum).

"It is indisputable that human activities have caused climate change and are causing extreme weather events to become more frequent and severe, affecting all regions of the planet," said IPCC chair Hyesong Lee.

"To use a sports simile, the atmosphere is 'doped', and we are now experiencing these events more frequently, as we have recently seen with the fires in Greece and California, or the floods in China and Europe," added World Meteorological Organisation (WMO) secretary-general Petteri Taalas.

The current changes in climate are "unprecedented" in recent centuries and even millennia, say the scientists who authored the report.

For example, according to the IPCC experts, the current atmospheric concentration of carbon dioxide, the main greenhouse gas, exceeds 410 particles per million, the highest in the last two million years.

The experts estimate in the report that, if the current rate of greenhouse gas emissions continues, global temperatures will rise by 2.7 degrees Celsius by the end of the century compared to the pre-industrial era average (1850-1900).

This increase, which would also lead to more extreme weather events such as droughts, floods and heat waves, would fall far short of the target of less than 2 degrees Celsius set by the Paris Agreement, which even called for limiting the rise to 1.5 degrees Celsius.

The new report by the world's leading climate change institution, delayed by several months due to the COVID-19 pandemic, considers five scenarios, depending on the level of emissions reached.

Maintaining the current situation, in which the global temperature is on average 1.1 degrees higher than in the pre-industrial period (1850-1900), would not be enough: the scientists predict that this would lead to a rise of 1.5 degrees in 2040, 2 degrees in 2060 and 2.7 degrees in 2100.

In the most pessimistic scenario, where emissions of carbon dioxide and other greenhouse gases would double by mid-century, the increase could reach catastrophic levels of around 4 degrees by 2100, the report warns.

Each degree increase could mean 7 per cent more precipitation worldwide, leading to an increase in storms, floods and other natural disasters.

Extreme heat waves, which occurred about once a decade in pre-industrial times and currently occur 2.3 times, could multiply up to 9.4 times (almost once a year) in a scenario with 4 degrees warmer.

By contrast, in the most optimal scenario considered by the report, one in which carbon neutrality (net zero emissions) is achieved by mid-century, the temperature increase would be 1.5 degrees in 2040, 1.6 degrees in 2060 and even lower to 1.4 degrees by the end of the century.

"Stabilising the climate will require strong, rapid and sustained reductions in greenhouse gas emissions to reach net zero emissions," said another co-chair of the expert group, Panmao Zhai.

The study, authored by 234 authors from 66 countries, acknowledges that emissions reductions would not have discernible effects on global temperature for about two decades, although the benefits on air pollution would be felt sooner, in a matter of a few years.

This paper, which focuses on the scientific basis of climate change, will be complemented in 2022 by two others produced by other IPCC working groups (one focusing on societal adaptation and the other on mitigation measures).

All three will serve to synthesise the IPCC's sixth comprehensive report, due in September 2022, to continue the work begun in 1990, 1995, 2001, 2007 and 2014.