Why does global warming cause extreme weather

Consistent with theoretical expectations, the types of events most closely related to temperature, such as heatwaves and extremely hot days, are becoming more likely. Heavy rainfall and snowfall events which increase the risk of flooding are also generally becoming more frequent. However, studies can show whether the warming climate made an event more severe or more likely to happen. A warming climate can contribute to the intensity of heat waves by increasing the chances of very hot days and nights.

Climate warming also increases evaporation on land, which can worsen drought and create conditions more prone to wildfire and a longer wildfire season. These short-term and regional variations are expected to become more extreme in a warming climate. This is supported by available observational evidence in the North Atlantic. Attribution is about working out if the likelihood or magnitude of a particular event happening now is different from what it would be in a world that was not warming.

A useful analogy — as explained in the first BAMS report in — is of a baseball player who starts taking steroids. But it is possible to say how the steroids have altered the likelihood that the player hits a home run, by comparing their current and historical performances. As the report put it:. Another important point is that in cases where attribution science finds that climate change is making a given type of extreme weather more likely, it does not necessarily follow that the chance of experiencing that kind of weather gets incrementally higher each year.

Natural variability means that there will still be ups and downs in the strength and frequency of extreme events. Finally, there is usually a level of confidence attached to attribution results. So, while two studies might both find a role for human influence in a given weather event, the signal may be stronger for one than the other.

For the purposes of this analysis, the attribution map does not distinguish between high- and low-confidence results, but users can click through to each study for more details. That there is a more divided set of results for extreme rainfall than for heatwaves could suggest several things. In other cases, an inconclusive result could reflect the fact that rainfall or flooding events are inherently more complex than heatwaves, with many ways for natural variability to play a role.

Human factors, such as land use and drainage, also play a part in whether heavy rain leads to flooding. Take the UK, for example. This raises another important point. When it comes to interpreting the results of event attribution studies, it matters what the question is. For example, a study asked whether recent wet summers in northwestern European were a response to retreating Arctic sea ice pdf, p The paper notes that, in a chaotic weather system, the complex dynamics of the atmosphere mean the size and path of a storm or heavy rainfall event has a large element of chance.

This can make it tricky to identify where climate change fits in, potentially underestimating its influence. Higher temperatures mean warmer seas, higher sea levels and more moisture evaporating into the atmosphere. These are changes that scientists can be more confident in, the authors write, and so should be the focus for attribution studies — rather than looking at changes to circulation patterns in the atmosphere.

For example, the paper reexamines an earlier study pdf, p15 that suggested climate change had reduced the chances of the five-day heavy rainfall event that hit north-east Colorado in September Trenberth and colleagues argue that while climate change might not have made the specific weather system that brought the rain more likely, it will have contributed to the sheer volume of moisture in the atmosphere. While attribution studies of heatwaves are generally more straightforward than storms — as they focus on thermodynamic influences — the type of question they are asking is still important.

The Russian heatwave in is a good example of this. One study looking at the severity of the event did not find a role for climate change.

Yet another one , which did find an influence, looked at the likelihood of the event. This apparent contradiction is tackled by a third study that reconciles the other two. It is also important to stress that the absence of evidence for a link to climate change is not the same as evidence of absence. In other words, it does not necessarily mean there was no human influence, just that a particular analysis did not find one.

This is why a single study should never be considered the final word on how climate change influences a given type of extreme weather. Capetonians queue for water at natural springs around the city during the water crisis, January This mixed bag of results reflects the inherent complexity of droughts.

And, again, the specific question matters. Conclusions about the role of climate change in a specific drought could depend on whether a study looks at temperature, precipitation or soil moisture, for example.

While much has been achieved in the field of extreme event attribution in a short space of time, scientists are constantly looking for ways to tailor their work to suit the people who might use it. One major goal since the early days of the field has been to expand extreme event attribution to cover a larger and more diverse geographical area.

Where in the world scientists can carry out attribution studies — and for what kind of events — will always be limited by the quality and availability of observed data and appropriate models. The attribution map highlights, for example, that there are relatively few studies of extreme weather in Africa and South America. But, at the moment, there is also a heavy leaning towards weather events that are local to the modelling groups, or that have a particular scientific interest.

Otto explains:. The UK, California and Boulder [in Colorado] are, therefore, studied much more than other parts of the world, but that does not necessarily make them places particularly impacted by climate change.

This means that while the studies carried out so far are indicative of the role climate change is playing in extreme weather around the world, they should not be considered representative of all types of extreme weather everywhere, says Otto. She tells Carbon Brief:. As well as expanding the science to cover different types of weather and more of the world, scientists are getting faster at turning the handle on extreme event attribution studies — sometimes crunching the numbers just days after an event has occurred.

The rapid studies included here are all — bar one — produced by the World Weather Attribution WWA initiative, described earlier. While the WWA individual rapid assessments are not individually peer-reviewed, they are conducted using methods that have been through the peer-review process.

As the BAMS report explains:. By conducting the analysis in the immediate aftermath of a weather event, these rapid studies provide almost-real-time information on the climate change influence, rather than having to wait many months for a formal study. In some cases, these rapid assessments are later published in peer-reviewed journals. In these cases, the formal study is included in the attribution map, rather than the initial analysis.

Right now, the Atlantic coast of the United States and the Gulf of Mexico are experiencing some of the highest sea level rise in the world, which, combined with record rainfall, has led to catastrophic flooding. Overall, winters are getting milder and shorter; but recent winters have brought intense snowstorms and record-breaking frost. While it may seem contradictory, climate change may be contributing to more extreme winter weather. As the warming atmosphere traps water vapor later and later into the year, that precipitation leads to heavier snowfall when the temperatures do drop.

Another factor is the rapidly warming Arctic, which some scientists believe is weakening the jet stream and causing disruptions of the polar vortex. The polar vortex refers to bands of wind and low air pressure near the North Pole, which normally lock cold air over Arctic. When those bands break down, icy air can escape south in the form of freezing winters. In , record-breaking snowstorms knocked out power for nearly 4. Americans across the political spectrum are feeling the urgency of our climate deadline and calling for action on a scale that matches the threat.

This fight to preserve a livable planet touches everyone. Together, we can drive transformative change in service of the earth and justice for its people. Our Stories. Take Action. A drought emergency has been declared in 41 of California's 58 counties. Across the globe, extreme weather is becoming the new normal. Destructive wildfires Deadly heatwaves and drought Record hurricanes Torrential rains and flooding Intense winter storms From season to season and year to year, weather events that were once rare occurrences are now increasingly commonplace.

Why is this happening? Wildfires burn longer and wider Larger fires in hot, dry years. Close Section. Extreme heat gets hotter Heat waves pose health risks and strain our energy system. Woodland Hills, Calif. Drought conditions persist Moisture evaporates from waterbodies and soil. A dried out lake stands near the Navajo Nation town of Thoreau on Jun.

Warmer temperatures drive increases in precipitation Areas that have historically trended toward heavy precipitation will get wetter. Hurricanes are becoming more intense Storm systems draw their energy from warm ocean water. A mother and her 3-week-old baby are ferried from their home amidst the floodwaters of Hurricane Harvey in