In media articles about the unprecedented flooding in Sydney, you will often hear the statement that for every 1°C of warming, the atmosphere can hold about 7% more moisture.
This figure comes from research conducted by French engineer Saadi Carnot and published 200 years ago this year.
Now we know there is more to the story. Yes, warmer climates have the capacity to hold more moisture. But condensation of water vapor to form mustard drops releases heat.This, in turn, could promote stronger convection associated with thunderstorms, which could then lead to significantly more rainfall. This means that the intensity of extreme rainfall could increase by as much as 7% per degree of warming. What we're seeing is that thunderstorms could increase at about double or triple that rate - about 14-21% more rain for each degree of warming.
Hurricanes are a major cause of extreme flooding around the world, contributing to Brazil's devastating floods that submerged hundreds of cities, and inundated Dubai's airport and streets.
For Australia, we helped develop a comprehensive review of the latest climate science to guide future flood preparedness.This showed that the growth rate due to global warming for hourly or shorter duration extreme rainfall was about 7–28%, and for daily or longer duration extreme rainfall was 2–15%. This is much higher than current flood planning standards, which recommend a typical increase of 5% per degree of warming. Why are storms important for extreme rainfall?
For a thunderstorm to form, you need something like a large temperature difference between lower and higher air masses to create moisture and instability in the air.
We generally associate thunderstorms with intense localized rainfall over a short period of time. However, what we are seeing now is a shift toward more intense storm rainfall, especially for shorter periods.Extreme rainfall events are also more likely to occur when storms are accompanied by other weather systems, such as lows along the east coast and intense low pressure systems near eastern Australia. Record flooding in Lismore in February 2022 killed several people due to extreme rainfall over several days, which was partly caused by a severe storm along the lower east coast.
Climate change increases extreme flood risk factors
The latest report of the Intergovernmental Panel on Climate Change (IPCC) states that: The frequency and intensity of heavy rainfall events has increased over most land areas since the 1950s, for which observational data are consistent with trend analysis (high confidence ), and human-induced climate change is probably the main cause.
This increase is particularly pronounced in extreme rainfall of short duration, such as that caused by hurricanes.
Why? In part, this is due to the 7% figure – warmer air is able to hold more water vapor.But this doesn't explain everything. Something else is also going on: condensation produces heat. So as water vapor turns into droplets, more heat becomes available, and the hot air rises by convection. In storms, excess heat promotes stronger convection, where warm, moisture-laden air moves upward.
This explains why hurricanes can now produce so much rainfall in our warming world.Just as water vapor condenses to cause rain, it also creates storms that supercharge heat.
We are seeing a very rapid rate of increase in rainfall in Australia in recent decades. The daily rainfall associated with thunderstorms has increased well beyond the figure of 7 – about 2-3 times more.
Hourly rainfall extremes have also increased in intensity at a similar rate.
What about very sudden, extreme rainfall? Here, the rate of increase could potentially be even higher.A recent study examined extreme rainfall events lasting less than an hour near Sydney, suggesting an increase of 40% or more over the past 20 years. Other evidence also shows rapid trends in the intensity of extreme rainfall. There are obvious ones, such as fine-resolution modeling.
To model complex climate systems, we need supercomputers. But still, many of our models for climate projections do not drill down to grit resolution smaller than about 100 kilometers.
While this may work well for large-scale climate modeling, it is not suitable for directly simulating hurricanes.This is because the convection processes required to form storms occur on a much smaller scale than this. There is now a concerted effort to perform more model simulations at a much finer scale, so that we can improve the modeling of convection.
Recent results from these very fine-scale models for Europe suggest that convection will play a more important role in triggering extreme rainfall, including combined storms, such as low pressure systems and thunderstorms in conjunction with other raining.
This matches Australian observations, in which there is a trend for increased rainfall from thunderstorms with other types of storms such as cold fronts and cyclones (including low pressure systems in southern Australia). Will this change our planning for flooding?
Evidence of supercharged storm rainfall has increased in recent years.Australia's current flood guidance recommendations, which influence the construction of infrastructure projects, are based on extreme rainfall increasing by only 5% for each degree of warming. Our research review showed that the real figure is significantly higher .
This means that roads, bridges, tunnels built for the 5% figure may not be prepared to handle the extreme rainfall we are already seeing from supercharged storms.
While Australia has become more aware of the links between climate change and bushfires, studies show we are less likely to make the connection between climate change and more intense storms and floods. This needs to change. We still face some uncertainties in linking climate change to an extreme rainfall event.But the bigger picture is not so clear: A warmer world poses a greater risk of extreme flooding, often driven by extreme rainfall from supercharged storms.
So what should we do? The first step is to take the impacts of climate change, the risks of storms and floods as seriously as we now take wildfires.
The next step is to incorporate the best available evidence into how we plan for these future storms and floods. We've already loaded the dice for ongoing human-caused climate change and more severe floods to come That is, unless we can quickly reduce greenhouse gas emissions.(talk) AMS
This figure comes from research conducted by French engineer Saadi Carnot and published 200 years ago this year.
Now we know there is more to the story. Yes, warmer climates have the capacity to hold more moisture. But condensation of water vapor to form mustard drops releases heat.This, in turn, could promote stronger convection associated with thunderstorms, which could then lead to significantly more rainfall. This means that the intensity of extreme rainfall could increase by as much as 7% per degree of warming. What we're seeing is that thunderstorms could increase at about double or triple that rate - about 14-21% more rain for each degree of warming.
Hurricanes are a major cause of extreme flooding around the world, contributing to Brazil's devastating floods that submerged hundreds of cities, and inundated Dubai's airport and streets.
For Australia, we helped develop a comprehensive review of the latest climate science to guide future flood preparedness.This showed that the growth rate due to global warming for hourly or shorter duration extreme rainfall was about 7–28%, and for daily or longer duration extreme rainfall was 2–15%. This is much higher than current flood planning standards, which recommend a typical increase of 5% per degree of warming. Why are storms important for extreme rainfall?
For a thunderstorm to form, you need something like a large temperature difference between lower and higher air masses to create moisture and instability in the air.
We generally associate thunderstorms with intense localized rainfall over a short period of time. However, what we are seeing now is a shift toward more intense storm rainfall, especially for shorter periods.Extreme rainfall events are also more likely to occur when storms are accompanied by other weather systems, such as lows along the east coast and intense low pressure systems near eastern Australia. Record flooding in Lismore in February 2022 killed several people due to extreme rainfall over several days, which was partly caused by a severe storm along the lower east coast.
Climate change increases extreme flood risk factors
The latest report of the Intergovernmental Panel on Climate Change (IPCC) states that: The frequency and intensity of heavy rainfall events has increased over most land areas since the 1950s, for which observational data are consistent with trend analysis (high confidence ), and human-induced climate change is probably the main cause.
This increase is particularly pronounced in extreme rainfall of short duration, such as that caused by hurricanes.
Why? In part, this is due to the 7% figure – warmer air is able to hold more water vapor.But this doesn't explain everything. Something else is also going on: condensation produces heat. So as water vapor turns into droplets, more heat becomes available, and the hot air rises by convection. In storms, excess heat promotes stronger convection, where warm, moisture-laden air moves upward.
This explains why hurricanes can now produce so much rainfall in our warming world.Just as water vapor condenses to cause rain, it also creates storms that supercharge heat.
We are seeing a very rapid rate of increase in rainfall in Australia in recent decades. The daily rainfall associated with thunderstorms has increased well beyond the figure of 7 – about 2-3 times more.
Hourly rainfall extremes have also increased in intensity at a similar rate.
What about very sudden, extreme rainfall? Here, the rate of increase could potentially be even higher.A recent study examined extreme rainfall events lasting less than an hour near Sydney, suggesting an increase of 40% or more over the past 20 years. Other evidence also shows rapid trends in the intensity of extreme rainfall. There are obvious ones, such as fine-resolution modeling.
To model complex climate systems, we need supercomputers. But still, many of our models for climate projections do not drill down to grit resolution smaller than about 100 kilometers.
While this may work well for large-scale climate modeling, it is not suitable for directly simulating hurricanes.This is because the convection processes required to form storms occur on a much smaller scale than this. There is now a concerted effort to perform more model simulations at a much finer scale, so that we can improve the modeling of convection.
Recent results from these very fine-scale models for Europe suggest that convection will play a more important role in triggering extreme rainfall, including combined storms, such as low pressure systems and thunderstorms in conjunction with other raining.
This matches Australian observations, in which there is a trend for increased rainfall from thunderstorms with other types of storms such as cold fronts and cyclones (including low pressure systems in southern Australia). Will this change our planning for flooding?
Evidence of supercharged storm rainfall has increased in recent years.Australia's current flood guidance recommendations, which influence the construction of infrastructure projects, are based on extreme rainfall increasing by only 5% for each degree of warming. Our research review showed that the real figure is significantly higher .
This means that roads, bridges, tunnels built for the 5% figure may not be prepared to handle the extreme rainfall we are already seeing from supercharged storms.
While Australia has become more aware of the links between climate change and bushfires, studies show we are less likely to make the connection between climate change and more intense storms and floods. This needs to change. We still face some uncertainties in linking climate change to an extreme rainfall event.But the bigger picture is not so clear: A warmer world poses a greater risk of extreme flooding, often driven by extreme rainfall from supercharged storms.
So what should we do? The first step is to take the impacts of climate change, the risks of storms and floods as seriously as we now take wildfires.
The next step is to incorporate the best available evidence into how we plan for these future storms and floods. We've already loaded the dice for ongoing human-caused climate change and more severe floods to come That is, unless we can quickly reduce greenhouse gas emissions.(talk) AMS
