The hotter the world gets, the more water the air can hold and the more it will rain.

Last year’s disaster season saw catastrophic floods around the world caused by record levels of rainfall. After putting in the hottest year on record, this year is going to be worse, with sea temperatures already breaking all-time records – records that were only set last year.

The atmosphere already holds circa 10% more water vapour today than just 30 years ago. The warmer the atmosphere gets, the more water it can hold – about 7% more per 1°C of warming according to the Clausius-Clapeyron equation – and scientists have already observed a significant increase in atmospheric moisture, reports Climate Signals.

“Storms supplied by climate change with increasing moisture are widely observed to produce heavier rain and snow. Research indicates that the increase in atmospheric moisture is primarily due to human-caused increases in greenhouse gases [GHGs],” Climate Signals reports.

All the attention has been focused on CO2 emissions, with now rising awareness that methane emissions are also a big problem. But NASA points out that water vapour is actually the world’s most abundant GHG.

“It’s responsible for about half of Earth’s greenhouse effect – the process that occurs when gases in Earth’s atmosphere trap the sun’s heat. GHGs keep our planet liveable. Without them, [the] Earth’s surface temperature would be about 33C colder. Water vapour is also a key part of [the] Earth’s water cycle: the path that all water follows as it moves around Earth’s atmosphere, land and ocean as liquid water, solid ice and gaseous water vapour,” NASA said in a note on water vapour.

Make-up of the main atmospheric GHGs

Having said that, water vapour is not a driver of Earth’s current warming, but a consequence of it, explains NASA, although increased water vapour in the atmosphere amplifies the warming caused by other GHGs.

The global average temperature has risen by about 1.1C since the start of the industrial age in 1800 and that has led to more water being evaporated, says NASA. And the United Nations’ Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report states total atmospheric water vapour is now increasing by 1% to 2% per decade.

But the pace of water vaporisation is now accelerating rapidly. Last year was the first time that temperatures were 1.5C above the long-term industrial base line, which means as much as 10% more water is in the atmosphere than only a year earlier.

What is special about water vapour is that unlike the other five main GHGs – carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons – water will condense at lower temperatures, while the other five gases won’t. In simple terms, lower temperatures mean more rain and vice versa.

“This makes water vapour the only greenhouse gas whose concentration increases because the atmosphere is warming, and causes it to warm even more,” says NASA.

As water vapour is rapidly increasing thanks to the accelerating rate of global warming, scientists worry that increases in vaporisation could turn into one of the runaway vectors of global warming that will heat the earth faster than the current models predict.

More water in the air will also fuel more storms and knock the global water cycle out of kilter. Wet regions will get wetter and dry regions drier. The more water vapour that air contains, the more energy it holds. This energy fuels intense storms, particularly over land, and causes more extreme weather events.

Spring has barely arrived, yet northern Italy has already experienced extreme water-related weather. Heavy rainfall, snow and strong winds have been affecting northern Italy since 26 February, causing snow avalanches, flooding and landslides, resulting in at least 11 deaths.

But more evaporation from the land also dries soils out. When water from intense storms falls on hard, dry ground, it runs off into rivers and streams, causing flash floods instead of dampening soils. Ironically, the more water there is in the air increases the risk of drought in drier lands, which as bne IntelliNews reported, is already afflicting the arid lands in Central Asia.

While Northern Europe is already seeing dramatic increases in water vapour and hence can expect much more rain, Southern Europe is already on course to see a repeat of last year’s baking 40C-plus temperatures that will cause widespread droughts.

 

As bne IntelliNews recently reported, the changing patterns of rainfall are going to lead to extreme water stress by 2050 in large swaths of sub-Saharan Africa, the whole Arabian peninsula and India, and scientists predict that 3bn people will live in zones that will have been made uninhabitable by 2070.

The situation will be made even worse if the AMOC (Atlantic Meridional Overturning Circulation), currents that carry warm water from the equator to the northern seas, collapses and will cause a mini-ice age in Northern Europe when it stops flowing. The collapse of the AMOC is predicted to happen with 95% certainty sometime before 2050 and is one of the environmental tipping points that will have unpredictable environmental consequences. That will shift the rain patterns over the Amazon; the Amazon River was already drying up last year and water levels in the Panama Canal also fell to record lows. The melting of Russia’s permafrost is another tipping point that is probably less than a decade away.

Raining cats and dogs

The map below shows projected percentage change in precipitation between the current climate (represented by the 1981-2000 average) and the end of the century (2081-2100) as an average of all of the climate models featured in the latest IPCC report, using the high-end warming scenario. Purple shows areas where precipitation will increase, orange indicates less future rain and snow, and the dots indicate where nine out of ten climate models agree.

Global precipitation pattern changes by 2081-2100 - IPCC

“The average of the models shows large increases in precipitation near the equator, particularly in the Pacific Ocean. They also show more precipitation in the Arctic and Antarctic, where cold temperatures currently limit how much water vapour the air can hold,” a note by Carbon Brief explained. “The Mediterranean region is expected to have around 20% less precipitation by 2100 in a [high end 4C warming assumption] world, with similar reductions also found in southern Africa. Western Australia, Chile and Central America/Mexico may all become around 10% drier.”

However, a lot of uncertainty surrounds the changing rainfall predictions. The above chart is based on an averaging of the 39 different climate models used by IPCC and many of them give very different predictions of what will happen to rainfall patterns over the long term. In at least one model much of the world outside high-latitude areas and the tropical oceans shows sizable drying and there is at least one model where nearly any given location in the world gets wetter.

But there is widespread agreement among the models that both the tropical Pacific and high-latitude areas will have more precipitation in the future. India, Bangladesh and Myanmar will all become wetter, as will much of northern China.

The models largely agree that the Mediterranean region and southern Africa will have less precipitation in the future. They also agree on reduced precipitation in southwest Australia around Perth, in southern Chile, the west coast of Mexico and over much of the tropical and subtropical Atlantic ocean.

Interestingly, despite all the focus on drought in the state of California, there is no consensus among climate models that the region will experience less precipitation on average in a warmer world.

Models also generally agree that precipitation, when it does occur, will become more intense nearly everywhere. Unlike average annual precipitation, almost the entire world is expected to see an increase in extreme precipitation as it warms. The science suggests that while a one degree rise in temperature leads to a 7% increase in water vapour, it also results in a 15% increase in extreme rain, especially at higher elevations, according to research published in Nature.

Models suggest most of the world will have a 16-24% rise in heavy precipitation intensity by 2100. In other words, heavy rain is likely to get heavier. In the map red areas show decreases in heavy precipitation, while blue areas indicate increases.

Global precipitation increases in heavy rainfall per degree of warming by 2100