Across China and Western Europe in July, the amount of rain that might typically fall over several months to a year came down within a matter of days, triggering floods that swept entire homes off their foundations. In June, the usually mild regions of Southwest Canada and the US’s Pacific Northwest saw temperatures that rivaled highs in California’s Death Valley desert. The severe heat was enough to buckle roads and melt power cables.
Yesterday, a landmark United Nations report helped put those kinds of extreme events into context. By burning fossil fuels and releasing planet-heating greenhouse gases into the atmosphere, humans are fueling more dangerous weather. Researchers have been able to connect the dots between greenhouse gas emissions and climate change for decades. But the new report showcases a big leap forward in climate science: being able to tie the climate crisis directly to extreme weather events like the June heatwave, which would have been “virtually impossible” without climate change according to recent studies.
The Verge spoke with Alex Ruane, one of the authors of the new report and a research physical scientist at the NASA Goddard Institute for Space Studies. He walks us through the phenomena that’s supercharging extreme weather events. And he explains why scientists have gotten so much better at seeing the “human footprint” in each weather disaster.
This interview has been lightly edited for length and clarity.
The new United Nations report ties many changes in extreme weather to a more intense water cycle. What is the water cycle and how does it affect the weather?
The water cycle is basically the way that we track moisture moving through the climate system. So it includes everything from the oceans to the atmosphere, the clouds, ice, rivers, lakes, the groundwater, and the way that those things move and transfer moisture and water from place to place.
So when we’re talking about the intensification of the water cycle, we’re basically saying things are moving faster. Air is pulling the moisture out of the oceans and out of the land faster. It’s moving more moisture from place to place on the planet. And when it rains, it can come down hard.
The fundamental difference is that there is more energy in the system. There’s more heat. And as the temperature goes up, there is an overall increase in the amount of moisture that the air is trying to hold. So that means when a storm happens, there’s more moisture in the air to tap into for a big, heavy downpour. It also means that when air moves over a region, it has the potential to suck more moisture out of the ground more rapidly. So the same phenomenon is leading both to more intensive rainfalls and floods and precipitation, and also to more stark drought conditions when they do occur.
How are people affected by those changes?
So, I personally live in New York City. We are affected by the water cycle, for example, when there’s a heavy downpour it can flood subway stations. It can lead to surface flooding in rivers and streets that can affect transportation.
Heavy rain inundated New York City yesterday afternoon, partially submerging the 157th St Subway station in Washington Heights, footage shows.— NPR (@NPR) July 9, 2021
A flash flood watch was in effect for New York City through Thursday night. pic.twitter.com/VqbyR3hM2q
Other parts of the world have different engagements with the water cycle. They may be concerned about the snow fall or river floods that affect broad areas. And then of course huge parts of the world are concerned about drought. When we look at something like drought, it doesn’t just affect agriculture. It also affects ecosystems and urban parks. It affects water resources and infrastructure like power plants and roads and buildings.
So in all of these climate factors, we see that more than one sector is affected by these changes. We also see that if you take any specific thing that we care about, like agricultural fields, they are affected by more than just one type of climate change.
A specific set of climate conditions can lead to two extremes at the same time. So for example, heat and drought often go together because as conditions become drier, all of that sunshine, all of that energy, all of that heat goes into warming the air. That is a reinforcing cycle that can make hot and dry conditions even more extreme.
The big picture, as we’re seeing it, is that climate change is affecting all of the regions on Earth, with multiple types of climate changes already observed. And as the climate changes further, these shifts become more pronounced and widespread.
I’ve read that “weather whiplash” is becoming more common because of climate change — what is “weather whiplash”?
This idea that you can go from extreme to extreme very rapidly is giving society this sensation of a whiplash. This is part of the idea of an intensified water cycle. The water is moving faster, so when a wet condition comes it can be extremely wet. And then behind it could be a dry condition that can quickly get extremely dry. That type of shift from wet to dry conditions is something that we explore and understand in our climate models, but the lived experience of it can be quite jarring — and not just uncomfortable, but a direct challenge for ecosystems and other things that we care about in society. They really are connected in many cases to the same types of phenomenon, and this new report connects the dots between this phenomenon and our human footprint.
How do scientists study how climate change affects extreme weather events?
There have been big steps forward in the methodologies and the scientific rigor of detection and attribution studies, which is another way of saying: understanding the human influence on these events.
The basic idea behind the extreme event attribution is that we need to compare the likelihood that an event would have happened without human influences against the likelihood of that event happening, given that we have influenced the climate.
We are able to use observational records and our models to look at what conditions were like before there was strong human influence. We look at what we call a preindustrial condition, before the Industrial Revolution and land use changes led to greenhouse gas emissions and other climate changes.
If we can understand how likely events would have been before we had our climate influences, and then compare it against the likelihoods today with those climate change influences factored in, that allows us to identify the increased chance of those events because of our influence. It allows us to attribute a human component of those extreme events.
How have researchers gotten so much better at attributing extreme weather events to climate change?
This is a really exciting, cutting-edge field right now.
Methodological advances and several groups that have really taken this on as a major focus of their efforts have, in many ways, increased our ability and the speed at which we can make these types of connections. So that’s a big advantage.
Every year, the computational power is stronger in terms of what our models can do. We also use remote sensing to have a better set of observations in parts of the world where we don’t have weather stations. And we have models that are designed to integrate multiple types of observations into the same kind of physically coherent system, so that we can understand and fill in the gaps between those observations.
The other thing, of course, is when you look at any single attribution study, you get a piece of the picture. But what the new report does is bring them all into one place and assesses them together, and draw out larger messages. When you look at them all together, it is a much stronger and more compelling case than any one single event. And this is what the scientific community is showing us, that these things are part of a larger pattern of change that we have influenced.
What should we expect in the future when it comes to extreme weather? And what might we need to do to adapt?
First of all, it’s not like drought is a new phenomenon. There are parts of the world that are dealing with these conditions every day of the year. What we’re seeing, however, is that the overall set of expected conditions is moving into uncharted territory.
I want to emphasize it’s not just the record levels that we care about. We also care about the frequency by which these extremes occur, how long they last, the seasonal timing of when things like the last frost occurs, and also the spatial extent of extreme events — so where are conditions going to happen in the future that are outside of the observed experience of the last several generations.
It is a set of challenges that we have to face in terms of how do we adapt or manage the risk of these changes. Also, how do we prepare knowing that they may come in combination or in overlapping ways, with more than one extreme event happening at the same time, or in the same season in a sequence, or potentially hitting different parts of the same market or commodities trade exchange or something like that.
We are facing a situation where we have more information about these regional risks, but also know that every increment of climate change that occurs makes these changes more prominent. That sounds scary, but it also gives us agency. It gives us the ability to reduce these changes if we reduce emissions, and if we can eventually limit them to something like net zero — no total carbon emissions into the climate system. And in that sense, I still remain optimistic despite all this information that you’re seeing in the report about the changes that could come. The bottom line is we have the potential to reduce those changes, if we can get emissions under control.