24th May 2018

Global warming predicted to reach 4°C by 2084

Researchers in China have published a new analysis that shows the Earth's climate will likely increase by 4°C, compared to pre-industrial levels, towards the end of the 21st century.

To understand the severity of this, consider that the Paris Agreement of the UN – a global effort by nearly every country on the planet (the USA is the only nation to withdraw) – seeks to limit the increase to 2°C. Above this level, it is generally agreed by climate scientists that the impacts will become catastrophic and beyond the ability to control. A rise of 4°C was described by the UK's leading climate research institution as "incompatible with an organised global community; likely to be beyond ‘adaptation’, devastating to the majority of ecosystems, and [with] a high probability of not being stable."

"A great many record-breaking heat events, heavy floods, and extreme droughts would occur if global warming crosses the 4°C level, with respect to the pre-industrial period," says Dabang Jiang, senior researcher at the Institute of Atmospheric Physics of the Chinese Academy of Sciences. "The temperature increase would cause severe threats to ecosystems, human systems, and associated societies and economies."

The analysis by Jiang and his team appears in the July 2018 issue of the journal Advances in Atmospheric Sciences.

For their study, the team used a trend line called RCP8.5 – the path humanity is currently on. This worst-case scenario assumes that greenhouse gas levels will keep rising unabated, as they are doing right now. They compared 39 models from the Coupled Model Intercomparison Project, which develops and reviews different climate models to ensure the most accurate simulations possible.

They found that the models varied in their projections for an increase of 4°C from as early as 2064, to as late as 2095. Of the 39 models, 10 did not specify any date at all. The median average year was 2084.

A global temperature rise of 4°C, without mitigation, would have devastating and long-lasting impacts. According to the National Research Council (NRC), part of the U.S. National Academy of Sciences, nine out of ten summers would be hotter than the hottest ever experienced during the late 20th century over nearly all land areas.

Rainfall intensity would increase by up to 40% during the heaviest precipitation events, damaging plants and exacerbating soil erosion leading to further stores of carbon being released. Although CO2 is plant food, excessive temperatures can damage vegetation and a global average rise of 4°C could mean yield losses of between 20 and 60% for crops as currently grown. The area burned by wildfires in parts of the western United States could increase tenfold.

While hurricane frequency would likely decrease by 20%, the intensity of each storm system would rise by up to 16% and their destructive power (cube of the wind speed) would increase by up to 48%, according to the NRC. Jet streams and storm tracks would generally shift closer toward the poles.

Every credible scientific study concludes that sea levels – already rising rapidly – will continue to rise throughout the 21st century, resulting from a combination of thermal expansion and the estimated 1 trillion tons of melting land ice entering the oceans each year. By 2084, assuming a global temperature increase of 4°C, between 1 and 1.5 m (3.2 to 4.9 ft) of additional sea level is likely, but even this may be underestimated, due to feedbacks not accounted for in previous models. Many coastal cities will be devastated by this change, with trillions of dollars' worth of real estate put at risk. It is almost inevitable that low-lying regions such as the Maldives will have to be abandoned; however, even wealthy cities like Miami could share this fate. Alongside the rise in sea levels will be a dramatic increase in ocean acidification, jeopardising shell-based marine life and wiping out coral reefs that are a source of income and tourism for half a billion people.

In the very long term, over timescales of centuries or millennia, deep ocean sediments would release additional carbon, while the Greenland ice sheet could disappear entirely, adding a further 7.5 m (24 ft) to sea levels. Ultimately, even the Antarctic ice sheet could melt too, adding a massive 61 m (200 ft).

Jiang and his team continue to investigate the changes associated with 4°C of global warming in extreme climates.

"Our ultimate goal is to provide a comprehensive picture of the mean and extreme climate changes associated with higher levels of global warming based on state-of-the-art climate models, which is of high interest to decision-makers and the public," he concludes.

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