‘In rain, as in wealth, the rich are getting richer and the poor are getting poorer,’ wrote Columbia University research scientist Michela Biasutti last year in Natural Geoscience.
Yet, unlike with money, more rain often doesn’t equate to an improved situation; rainfall extremes – whether too much or too little – can have a devastating effect, on people, ecosystems, even landforms. And changes in rainfall patterns are among the least understood, yet most critical, aspects of the world’s changing climate.
According to an increasingly large body of research, water-rich regions are getting wetter, while drier areas are becoming even drier. Scientists say that the effects of these changes are already visible, with increased incidences of large-scale flooding, droughts and natural disasters linked to too much or too little rainfall. ‘There’s very solid evidence that the mean state is changing, and there’s also evidence showing that there could be more variability – more floods, more droughts – attached to that new norm,’ says Kate Marvel, a climate scientist at the Lawrence Livermore National Laboratory (LLNL) in California.
In 2012, there was record or near-record flooding in Central and Eastern Europe, with the Danube rising to 12.8 metres, its highest recorded level since 1501. That same year, across Canada, flooding forced about 100,000 people from their homes, causing damage estimated to be as high as CAD$6billion. The damage in Europe was estimated to be in the region of €12billion.
Last year, in Australia, record-breaking temperatures, coupled with scant rainfall, saw fires rage in New South Wales for more than a week, the most devastating fires the state had experienced in decades. By the morning of 18 October, more than 100 fires were burning across the state, eventually razing more than 100,000 hectares of land and hundreds of homes. The conflagration evoked memories of Victoria’s 2009 fires, where a similar combination of high temperatures and low rainfall helped to fuel fires that burnt out more than 450,000 hectares of land, destroyed more than 2,000 homes and killed 173 people.
‘From the data we have, we can clearly see an increase in the frequency and intensity of weather-related loss events,’ says Peter Höppe, head of geo-risks research at Munich Re, one of the world’s largest reinsurance companies (they insure the policies of other insurance companies). ‘What we’re seeing today gives us a clue as to what we can expect in the next decade or two.’
As far back as 2007, a study found that globally, the number of large inland flood catastrophes over the ten-year period to 2005 was twice as large per decade as between 1950 and 1980, and that the related economic losses had increased by a factor of five. Similarly, a February 2011 report in Nature found that the overall likelihood of incidents of extreme rainfall had doubled due to rising levels of greenhouse gases. A second report, published in the same issue of the journal, suggested that in the Northern Hemisphere, the possibility of there being heavy precipitation on any given day rose by about seven per cent during the second half of the 20th century.
There have always been dry, desert regions and wet, tropical regions on the planet, but according to scientists, these regions are gradually expanding. They’ve linked this development directly to the gradual warming of the sea and land since the Industrial Revolution. These increased temperatures lead to warmer air, which, in turn, absorbs more water vapour. This tends to result in more extreme rainfall events, especially in tropical regions.
A recent review of global rainfall led by researchers at the University of Adelaide and published in February last year, using data compiled from more than 8,000 weather stations around the world, concluded that for every degree increase in global atmospheric temperature, there was a seven per cent increase in extreme rainfall intensity. ‘If the global average temperature increases as expected by 3°C–5°C by the end of the 21st century, this could mean a very substantial increase in rainfall intensity,’ says Dr Seth Westra, the paper’s lead author.
The study also demonstrated that while the strongest increases in extreme rainfall intensity have occurred in tropical regions, most of the weather stations showed an increase.
Last November, scientists at the LLNL published a study that compared climate model predictions with 33 years’ worth of satellite weather data. They concluded that natural variability (such as that caused by El Niño and La Niña events) couldn’t account for the observed changes in global precipitation patterns. ‘You can’t explain this through any known mode of climate variability, but it’s consistent with what we expect global warming to be doing,’ says Marvel, who was the study’s lead author.
But while tropical regions are experiencing increased rainfall, parts of Africa that have long struggled with drought are receiving even less rainfall. In a report published in 2011 in Climate Dynamics, scientists from the US Geological Survey (USGS) and the University of California, Santa Barbara, determined that in eastern Africa, a climate-change-related decrease in rainfall was putting 17.5 million people at risk. They suggested that the increased frequency of drought observed in eastern Africa, one of the world’s driest regions, over the past 20 years was likely to continue as long as global temperatures continued to rise.
‘It isn’t all of East Africa, but rather in places such as Nairobi and southern Somalia. Unfortunately, it just happens to coincide with the most food-insecure population on the planet – people who struggle to get enough food even in a good year,’ says Dr Chris Funk, a USGS climatologist who co-wrote the study.
Eastern Africa is particularly susceptible because over the past century, the Indian Ocean has warmed especially rapidly. The resulting warmer air and increased humidity has produce more frequent rainfall over the tropical Indian Ocean. At the same time, an easterly flow of dry air is pushed towards eastern Africa, which suppresses air movement over the region, decreasing precipitation during the so-called ‘long-rains’ season of March–June, leading to a greater frequency of drought in places such as Ethiopia and Kenya. ‘With the research we’ve done, what we find is that the western Pacific weather tracks almost perfectly with the IPCC predictions – that rainfall is going up and up there – and that poses a real threat to eastern Africa,’ Funk says.
Water availability isn’t just a matter of rainfall. In many parts of the world, communities rely on glacial meltwater for their water supplies. A combination of increased temperatures and decreased snowfall has sent many of these glaciers into retreat, setting alarm bells ringing for the villages, towns and cities that get their water from the lakes and rivers they feed.
This is particularly true in South America. The Andes are home to 70 per cent of the world’s tropical glaciers, and some 30 million people rely on the water these glaciers produce for drinking, agriculture and hydropower. For example, Quito in Ecuador and La Paz in Bolivia draw half and 30 per cent of their water supplies respectively from the Antizana and Cotopaxi glacier basins.
But according to a study published in the Cryosphere last year, Andean glaciers have shrunk by between 30 and 50 per cent since the 1970s. The glaciers are retreating at their fastest rates in more than 300 years, and many could disappear within the next few years, particularly those located at lower altitudes.
Climate change researchers all agree that accurately predicting future changes to rainfall patterns is extremely difficult. ‘Climate models aren’t very good at doing rainfall,’ says Westra. ‘Rainfall is a very local process, and is affected by things such as local mountain ranges or coastal effects; where I live in Australia, I can drive ten kilometres away and double the amount of annual rainfall.’ Marvel agrees: ‘Temperature changes are easy to gauge, but precipitation has to do with so many different factors.’
Even pinning the blame for weather-related natural disasters, especially when they’re related to rainfall – be it flood or drought – on anthropogenic climate change is generally impossible. ‘I think you can never say with certainty any one event is caused by climate change,’ says Marvel. ‘But the heatwaves in Australia, with the bushfires that devastated the country last year and look like repeating this year, while we can’t definitively say that these were due to climate change, we can say that this is what climate change would look like.’
For the past few years, the American Meteorological Society has published an annual paper or supplement in which climate scientists attempt to explain the previous year’s weather extremes from a climate perspective. ‘They’re looking at the extreme climate events of that year and looking at the question of whether there is a reliable fingerprint of climate change,’ says Funk. ‘The science is new, but I think this is going to help people see that these things are actually affecting us already. It’s an exciting development.’
The one thing on which most climate scientists agree is that the global water cycle is changing, and that different regions will be hit in different ways. Dry regions can expect to get drier and wet regions are going to get wetter. ‘We have rainy tropics and dry, subtropical desert zones. Going forward, we can expect those zones to expand outwards towards the poles,’ says Marvel. ‘As these deserts expand polewards, it’s a major problem, especially to nearby communities.’
This story was published in the March 2014 edition of Geographical Magazine