When global temperatures rise, they give more heat energy to the atmosphere. This in turn can make droughts and wildfires worse and lead to flooding when it rains.
Scientists are studying how these extreme events may be linked to climate change. To understand this emerging field, Carbon Brief has mapped every attribution study on the subject to date.
Climate Change
For decades, engineers, land-use planners and risk managers have used thermometers, rain gauges and satellite data to calculate the probability of extreme weather events. But a warming planet is making those events more frequent and intense – with consequences felt throughout the country.
The human-caused rise in greenhouse gases traps heat and warms Earth’s air and oceans, causing the water cycle to shift, changing weather patterns, and melting land ice. It’s also increasing the strength of storms, affecting their size and where they form, and increasing the amount of rainfall associated with hurricanes and other tropical cyclones.
Warming temperatures are also boosting sea level, which increases the impact of coastal storms and puts more stress on freshwater supplies during droughts. The warmer atmosphere also holds more water vapor, which leads to more frequent and severe flooding, especially in urban areas with poor drainage.
A growing number of climate-related disasters are occurring in the United States, from hotter summer temperatures to more frequent and severe wildfires. But it can be challenging to attribute any single event to global warming because many factors, like natural climate variability and regional variations, can affect the odds of a particular weather event.
A new study from Stanford researchers, however, reveals that the common scientific approach to predicting the likelihood of an extreme event by analyzing how frequently similar events occurred in the past can significantly underestimate those chances. The findings can improve how scientists assess and incorporate global warming into predictions of future extreme weather.
Weather Patterns
The Earth’s rising temperatures can intensify extreme weather events, such as heat waves, heavy rain and floods, and droughts. Scientists can confidently attribute the increase in these events to human-caused climate change, but it is difficult to pinpoint the exact cause of individual events.
To understand how global warming influences extreme events, scientists use a combination of history and models. They compare observations from Earth, air, sea and space to the results of climate models that simulate how the planet’s climate changes over time. This is called event attribution. Scientists also look at human activities that can amplify the impact of extreme weather, such as urban planning, wetland destruction and building homes in floodplains.
For example, rising ocean temperatures can make category 3 and higher Atlantic hurricanes more severe because they absorb more energy from the warm waters. Global warming can also make rainfall more intense, which can lead to more flooding and land erosion.
Researchers have conducted hundreds of event attribution studies for the most common types of extreme weather. This interactive map shows how many of these studies find that human activity has made the event more likely or severe (red), less likely or severe (yellow) or no influence at all (blue). Click on a circle or hexagon to see the number of studies in each category.
This map is updated annually to include new studies. The dotted lines show the range of confidence in the findings: high confidence means that the finding is very likely due to human activity, medium confidence means that the finding is likely due to human activity, and low confidence means that the finding is not very likely or may be inconclusive.
The map includes studies of the three most common types of extreme weather: heat waves, heavy rain and flooding, and droughts. Studies of 152 extreme heat events found that human activity made the event more likely or severe, while only one study of a drought found no effect. This year’s study of 126 flood events and 85 droughts showed similar results. Scientists have less confidence in the effects of climate change on extreme cold events, and they are inconclusive about the effects of changing sea levels on storm surges and coastal flooding.
Weather Forecasting
Weather forecasting has evolved from a manual process involving hand-counted thermometers and rainfall gauges to computer-based models that take many atmospheric factors into account. But human input is still required to select the most appropriate model and evaluate its accuracy. This requires pattern recognition skills, knowledge of the climate system, teleconnections and experience evaluating model performance over time.
Weather experts are able to save lives by alerting people when dangerous conditions are coming and giving them the chance to prepare for what’s ahead. They also help governments at the local, state and national levels understand what areas are under threat so that they can prepare accordingly and make sure essential services are available to residents who need them.
But a growing body of evidence shows that global warming is making extreme weather events more likely. And that’s creating new risks for people and the economy. For example, sea level rise makes it more likely that more coastal storms will produce flooding, while warmer temperatures cause land ice to melt, which in turn can add water to the world’s oceans.
Warming also contributes to drier conditions, as moisture evaporates more easily from soil and water bodies. This can lead to wildfires, and it can increase the intensity of droughts, as seen in California this summer.
Engineers, land-use planners and risk managers have long used historic weather records to calculate the probability of certain extreme events. But a new Stanford study reveals that not accounting for the influence of climate change when predicting future events can significantly underestimate their likelihood, with potentially devastating consequences for humans. This is because the lower atmosphere is becoming warmer and moister due to greenhouse gas emissions, a factor that can make some extreme events more likely.
Climate Models
Scientists use computer models to create simulations of our climate – everything from how moisture evaporates off the Earth’s surface and forms clouds, to where wind carries them and where rain falls. The interactions of these small-scale processes add up to the overall picture of our climate system, which includes how temperatures vary over time and place.
The models can help predict extreme weather by simulating a range of different scenarios, for example how the occurrence and intensity of hot days or heavy rainfall would be affected by human-caused climate change. For the most accurate results, scientists compare model output with real climate data from a number of sources. This process is called “bias correction,” and it’s a critical part of the modelling process, according to Maraun.
As we’ve seen in the devastation from wildfires and floods across western Europe, the window of predictability for extreme weather is shrinking as our planet warms. And that’s a problem, because engineers, land-use planners and risk managers use the frequency of extreme events to estimate the likelihood of costly impacts such as heat waves or flooding.
A recent study by Stanford University professor Noah Diffenbaugh found that a common scientific approach to estimating the odds of such events, based on historical observations, can significantly underestimate the effects of global warming. By analyzing how frequently extreme events occurred in the past and comparing them with future predictions, the researchers found that even small increases in global warming amplify the frequency of hot spells and downpours.
In contrast, models that take the rate of climate change into account perform better at predicting temperature trends over time. The higher resolution of some newer models – down to grids 2 kilometres squared – may also help them get more accurate with regional extremes, such as the heatwave that hit North America last summer.
It’s important to remember, though, that the current extremes we are experiencing were already predicted by climate models, and the predictions will only become more accurate as our understanding of the climate system improves. And as we continue to reduce emissions, models will be able to better predict how global warming affects the chances of hot days and heavy rainfall in the future.
