Sweltering heat waves could become the new normal, according to new research out of Stanford University that estimated the start of permanently hotter summers in parts of the world within two decades.
Noah Diffenbaugh, an assistant professor in Stanford's Department of Environmental Earth System Science and co-author of a forthcoming paper on the topic, recently spoke with me about the phenomenon. Below are excerpts from our interview.
On how this research came to be:
There's been a lot of emphasis on understanding global warming and what global warming does to other aspects of the climate system, such as temperature extremes. When I talk to the public about that work, I often get asked: 'Are you saying we're going to have heat waves all the time? Are the hottest conditions in an area going to become the new normal?' I find myself in a situation where I'm saying, 'That's not the study we did. We can't make that conclusion based on what we did.'
The interaction with the public inspired us to ask that question scientifically. There was so much interest in whether global warming caused what used to be the hottest conditions to become the normal conditions. We took it a step further and asked, 'Does global warming cause what was the hottest summer that ever occurred to become so common that there's never a summer that's cooler?' That was the impetus.
On how the research was conducted:
We used a combination of historical observation from direct measurements from weather stations around the globe and climate model experiments. We face a challenge in understanding global warming and climate change. We can't take the climate system and stick it in a lab and run controlled experiments on it. We have to use mathematical models based on the fundamentals of physics. We use these models to run experiments. We can proscribe as an experiment the known evolution of greenhouse gases over the 20th century and the climate models will calculate the temperatures of given areas across the globe in response. When we look at simulations of the historical period and compare them with observational records, we find that the climate models capture what's seen in the observations very well. With the climate models, we can proscribe changes in the greenhouse gas concentrations that could occur in the future and run experiments.
On the potential impact in different parts of the world:
The tropics show the most rapid and robust emergence of this extreme heat regime. We see that in the observational records. When we look forward with increasing greenhouse gas concentrations, we also see that the tropics are the earliest to show this emergence. This is a little bit counter to a lot of the narrative we're used to hearing. A lot of people say, 'I thought the poles were warming up fastest.' That's also true. In this case, we're not just looking at the magnitude of warming. We're looking at how quickly an area moves entirely to a warmer regime. That's a function of the magnitude of warming, but also how variable that old regime was.
Even though the tropics show the most robust emergence, we also see the emergence of extreme heat outside the tropics. In the northern hemisphere, we find that large areas of the United States and Europe and China all show this emergence over the next half century. There's a likelihood that many tropical areas will move into this new permanent heat regime within the next two decades. The western U.S. and the eastern seaboard show large areas that have a chance of moving into this extreme heat regime within the next four to five decades.
On the potential consequences of these temperature shifts:
We are acutely aware of the stress that comes from extreme heat: people dead from extreme heat, brownouts. We know from our day-to-day and historical experience that extreme heat can affect agriculture, energy supply and demand, human health and mortality. For any area, you can imagine what the impact might be based on that area's experience with extreme heat in the recent past. We can expect that if this does occur, people will have to deal with much more frequent stress from extreme heat.
On what can be done about this trend:
What the climate models project for the future is a product of the changes in greenhouse gas concentrations. Depending on where one looks globally, the projected permanent emergence of extreme seasonal heat could be avoided with a reduction in the increase in greenhouse gas concentrations. It's interesting that the most immediate intensification is in the tropics. We're looking at an emergence over the next two decades. It's very difficult to imagine slowing that pace of global warming over that time scale. It's mostly within the two degrees target that is agreed upon in the Copenhagen Accord.
On the next step in this research:
We have some more detailed simulations of the U.S. that we're moving forward with. There is certainly the potential for more detailed analysis on other parts of the world as well. This is one of the ongoing research avenues we're moving forward with.
This is something that hasn't been tackled before. It's a complex scientific challenge. It's important to keep in mind that while scientists will continue to debate issues such as are addressed in this paper, issues like this are out on the edges. The debate about whether or not this paper is correct doesn't mean that scientists don't agree on the basic physics of how greenhouse gases affect global mean temperature, for example.
Watch a video about the research.
Photo: Noah Diffenbaugh