The effects of climate change on individual extreme events consist of thermodynamic changes and atmospheric circulation changes. In a new opinion piece in Nature Climate Change, Kevin Trenberth, John Fasullo, and Ted Shepherd (TFS) argue that we should be focusing on the thermodynamic changes. While I agree that such an approach is useful and appropriate at times, I think Trenberth et al. take it too far.
Consider an extreme rainfall event, such as the rain that occurred in a few hours last May in the headwaters of the Blanco River in Texas and led to catastrophic flooding in Wimberley and San Marcos. The necessary ingredients are an ample supply of moisture, preferably throughout a deep column of the atmosphere, and a mechanism to produce focused, sustained ascent to condense all that moisture out of the atmosphere onto a small and vulnerable watershed.
There’s a limit to how much moisture can be in the air, and that limit is governed by temperature. That thermodynamic limit is analogous to the size of the pipe leading to a faucet. No matter what happens to the wind patterns, thermodynamics limits the amount of rainfall the atmosphere can produce.
Meanwhile, the wind patterns determine how much of that moisture is going to be delivered to a particular place at a particular time. They also determine how rapidly and efficiently that airborne moisture is going to be converted into rainfall. Those wind patterns are analogous to the handle on the faucet. No matter how much water is available, nothing will happen unless someone turns the handle, and the result depends on how far the handle is turned.
On our faucet, climate change affects both the pipe and the handle. As temperatures rise, the carrying capacity of the atmosphere for water vapor rises too, so the potential for very heavy rainfall increases. Meanwhile, climate change can turn the handle in different directions in different places, so that the net effect of global warming on very heavy rainfall at a particular location might be an increase, a decrease, or nothing at all.
TFS point out that “changes in the atmospheric circulation related to climate change are fairly small compared with natural variability”, that “forced circulation changes in climate models can be very non-robust, and physical understanding of the causes of these changes is generally lacking.” They argue that, because of this, studies of the role of climate change in circulation changes will generally be inconclusive. So they propose focusing on a question that’s easier to answer: given the particular weather pattern, how were the temperatures, precipitation and associated impacts influenced by climate change’s thermodynamic components?
In the case of our faucet, according to TFS, it’s hard to say what climate change is doing to the handle, so let’s focus on what it’s doing to the pipe.
I think there’s a lot of merit to this approach. I’ve used it many times myself, most recently when discussing May’s Texas rainfall. But rather than using it in all situations at all times, I would limit it to circumstances meeting the following two conditions:
- The size of the pipe appears to be an important controlling factor in the event.
- The effect of climate change on the handle is either small or not readily determined.
And one must always, always be clear that this is an “all else being equal” analysis, looking at just one component of the attribution question.
The Blanco flood was a case of very high precipitation efficiency. Essentially, all the moisture that the atmosphere could deliver was being dumped onto the Blanco River watershed. So it makes sense to talk about climate change’s effect on the size of the pipe. As TFS note, the observed 0.6 K increase in ocean temperatures over the past century or so leads directly to a 5% increase in the water vapor carrying capacity of the atmosphere. And in this case, the wind patterns were such that the atmosphere was carrying all the water vapor it could. So the thermodynamic component of this flood event was enhanced by about 5% by global warming.
We don’t know what climate change did to the handle of the faucet in this case. We do know that heavy rainfall like this has become more frequent over time, so it’s clear that whatever climate change is doing to the handle, it’s not cancelling out the effect of the bigger pipe.
That rain event was just one of many in May in Texas. Overall, Texas rainfall averaged 8.93” in May 2015, according to the latest numbers from the National Centers for Environmental Information (NCEI, formerly NCDC). This broke the previous statewide record by a staggering 2.27”. In the absence of climate change, the expected return period for such an event would probably be well over 1000 years.
But there weren’t many days in which the pipe running to Texas was filled to capacity. Instead, the unusual nature of the event seemed to be the unusual jet stream pattern apparently created by the robust El Niño event in the tropical Pacific Ocean. The key was not getting a full pipe, but keeping the handle turned on. Even if the pipe was only filled to half of capacity, Texas was going to get a lot of rain as long as the weather pattern held.
So the TFS approach is justified in some cases but not others.
For those who want to explore the issues in a bit more depth, let me now look at the general examples TFS give, and assess when their approach really is justified. My discussion will get a bit technical at times, so feel free to ignore unfamiliar terms.
Drought: “Given a drought, how was the drying (evapotranspiration) enhanced by climate change, and how did that influence the moisture deficits and dryness of soils, and the wildfire risk? Did it lead to a more intense and perhaps longer-lasting drought, as is likely?”
I’ve used that approach to understand the extraordinary drought in 2011 in Texas. Overall, it’s not clear whether rainfall will increase or decrease due to global warming. It’s also not clear whether La Niña (the proximate cause of the circulation changes associated with the drought) will become more or less common. So condition 2 is satisfied. In this case, the size of the pipe is the overall rise in global temperatures, which in turn have made Texas temperatures about 1 K warmer than they would have been otherwise, all else being equal. Thus global warming contributed to the intensity of the dryness through enhanced evaporation and contributed directly to the intensity of the heat wave.
Flood: “Given a flood, where did the moisture come from? Was it enhanced by high ocean temperatures that might have had a climate change component?”
I think that’s absolutely the wrong question. It conflates the overall rise in global ocean temperatures, which does have some spatial variability, with shorter-term, more cyclic ocean temperature variations. For example, suppose an event had its moisture source in the tropical eastern Pacific. It doesn’t matter, from a climate change causal perspective, whether eastern tropical Pacific temperatures were unusually warm, as with an El Niño, or unusually cold, as with a La Niña. What matters is, given an El Niño or a La Niña, ocean temperatures are 0.5 K warmer than they would have been a century ago. The observed sea surface temperatures are a product of natural variability and climate change, and for climate change’s role in an extreme event it’s only the climate change component of sea surface temperatures that matters.
Heat: “Given a heat wave, how was that influenced by drought, changes in precipitation (absence of evaporative cooling from dry land) and extra heat from global warming?”
From a thermodynamic pipeline perspective, the extra heat part is easy, as long as you can estimate the long-term temperature trend. When TFS speak of drought and changes in precipitation, though, they are referring climate change’s thermodynamic impact on drought and precipitation, which in turn has a thermodynamic influence on temperatures. This is too attenuated for me; it neglects too many possible circulation changes in the causal chain.
Snow: “Given extreme snow, where did the moisture come from? Was it related to higher than normal SSTs off the coast or farther afield?”
I have already explained why focusing on overall SSTs is the wrong approach. Even worse, though, is the fact that there’s a thermodynamic limit to snowfall: the freezing point. A cold atmosphere produces snow; a warm atmosphere produces rain. Suppose, without climate change, you have a temperature profile that maximizes atmospheric water content while still producing snow. Sure, if you raise the air temperature by 0.6 K, you’ll get a 5% increase in water content, but all the water (not just the extra 5%) will fall as rain instead of snow!
From a purely thermodynamic perspective, and assuming no change in the circulation pattern associated with the heavy snow event, global warming merely moves the location of the rain-snow line and thus the location of the heavy snow, without directly enhancing the heavy snow. Warm ocean temperatures may well intensify the storm’s circulation, but that’s precisely the effect that TFS want us to ignore.
The thermodynamic enhancement to snowfall is only applicable if the entire storm is cold enough to be a snow event. Thus it is legitimate to address the thermodynamic enhancement of snowfall in Antarctica, say, or in northern Canada in the wintertime. In warmer climates, though, it’s not so straightforward.
Storms: “Given an extreme storm, how was it influenced by anomalous SSTs and ocean heat content (OHC), anomalous moisture transports into the storm, ans associated rainfall and latent heating? Was the storm surge worse because of rising sea levels?”
TFS give an example of storm attribution with Superstorm Sandy. They note that a modeling study showed that artificially lowering SSTs in a broad strip along the coast cause the storm to be weaker and the precipitation to be greatly reduced. They then make the mistake of regarding this as at least partly equivalent to removing the thermodynamic effect of climate change. But it’s not so simple. Climate change has produced temperature changes in the coupled ocean-atmosphere system. If the numerical experiment alters temperatures in the ocean but not the atmosphere, it affects the extent to which the ocean is driving instability (both convective and moist baroclinic) in the atmosphere. These instability changes can have a much greater effect than any instability-neutral thermodynamic changes. So that part of their analysis is flawed.
The component of sea level rise induced by global warming is an unambiguous contribution to the severity of the storm surge. As with SSTs, it matters not whether sea level overall has risen or fallen at New York City, only what the global warming component to sea level change there has been.
The paper goes on to discuss other examples, focusing on the heavy rainfall in Colorado in September 2013. It takes direct aim at an attribution study that looked at model simulations of current and past climate and found that heavy rainfall in that area was less common in current climate simulations than past climate simulations, despite a simulated increase in maximum atmospheric water content.
I have much to criticize about TFS’s critique, but I’ll spare you the details. Two legitimate criticisms of the attribution study are that it used only one model, so the results are not necessarily robust across models., and that it examined simulations of a much broader-scale event than the one that occurred.
Still, though, suppose five years from now a study comes out with multiple models at higher resolution and finds the same overall result? Sure, the TFS approach is easier, but that’s no reason to abandon a more comprehensive analysis if such an analysis is possible.
In other words, don’t throw out the baby with the bathwater, even if the faucet is on.