Global warming escalates rainfall variability
A century of human-induced climate change has significantly increased rainfall variability over 75% of the Earth's landmass, with notable impacts in Australia, Europe, and eastern North America, according to recent research by Chinese scientists and the UK Met Office. Published in the journal Science, this study offers the first systematic observational evidence linking climate change to more erratic global rainfall patterns.
Climate models had previously predicted that climate change would exacerbate rainfall variability. However, this new research indicates that such variability has already escalated over the past century, especially in regions like Australia.
Prior studies typically focused on long-term average rainfall or extreme events, both of which present measurement challenges. This study, in contrast, zeroes in on rainfall variability, highlighting the irregular distribution of rainfall over time and location.
The results align with earlier research, underscoring that dry periods are becoming drier, while wet periods are becoming wetter. This increasing unpredictability raises the risk of droughts and floods, particularly concerning for countries like Australia.
Key findings
The study identifies a systematic increase in rainfall variability since the early 1900s, with global day-to-day rainfall variability increasing by 1.2% per decade. This trend has become more pronounced since 1950.
Increased variability means that rain distribution over time has become more erratic. For instance, a location might now receive a year's worth of rain in just a few days, leading to prolonged dry spells followed by intense downpours or rapid shifts from drought to flooding.
Analyzing observational data, researchers found that rainfall variability has risen across 75% of the studied land areas, particularly in Europe, Australia, and eastern North America. These regions benefit from detailed and long-term observational records. Other areas showed less prominent trends, potentially due to random variability or data inaccuracies.
The rise in daily rainfall variability was observed in all four seasons globally, with regional variations. The researchers attribute this trend mainly to human-caused greenhouse gas emissions, which have led to a warmer and more humid atmosphere, more intense rainfall events, and greater fluctuations between them.
These findings present new challenges for weather and climate forecasting, as well as for societal and ecological resilience and adaptation.
Impact of global warming on rainfall patterns
Understanding these findings requires a grasp of the factors influencing heavy rainfall in storms and how global warming affects these factors.
First, the amount of water vapor in the air is crucial. Warmer air holds more moisture, with each degree of global warming increasing average water vapor by 7%. Since the industrial revolution, Earth has warmed by 1.5°C, resulting in a 10% rise in lower atmosphere water vapor, thereby intensifying rainfall in storms.
Second, storm wind strength and the formation of large raindrops from smaller cloud particles also play roles. Although further research is needed to fully understand these factors, current evidence suggests they amplify rainfall increases during extreme storms while moderating them in weaker storms.
The new findings corroborate previous research on rainfall variability in Australia. Studies of extreme daily rainfall totals indicate that future increases in such events are likely to exceed past expectations, with the sharpest rises occurring during the most extreme events, nearly everywhere on the continent.
In 2022, an analysis of hourly rainfall data in Sydney revealed a 40% increase in maximum hourly rainfall over the past two decades. These findings have significant implications for Sydney's flash flood preparedness, as more intense downpours could overwhelm stormwater systems designed for previous conditions. However, it remains unclear how much of this increase is due to climate change and how widespread it is.
Increased rainfall variability also heightens drought risk. Climate models suggest that unless greenhouse gas emissions are swiftly reduced, rainfall variability will continue to rise in many parts of Australia. Even minor changes in variability can exacerbate drought conditions, making dry periods even drier.
Heeding the warning
Policymakers often focus on whether their regions are becoming wetter or drier overall. However, this new research highlights the need to prioritize variability. This increased volatility could lead to more severe droughts or dramatic spikes in extreme rainfall and flooding.
Such variability will pose significant challenges for governments and communities, from managing water resources to responding to natural disasters. Proactive preparation for these future challenges is essential.
As this global issue intensifies, the urgency to reduce greenhouse gas emissions and curb global warming becomes increasingly critical.