So Let’s Talk About the Science of How Climate Change Kicked Harvey into Higher Gear


Harvey is finally on the move.

After making a second landfall early Wednesday, the storm is passing slowly out of the East Texas region that has suffered so much first from Harvey’s initial lashing as a rapidly intensifying category 4 storm, and second from its long-lasting and unprecedented rainfall.

(Harvey rapidly intensifies into a category 4 monster just prior to landfall. This rapid intensification and other climate change related factors helped to make Harvey a more dangerous storm. Image source: NASA.)

At this point we can take a bit of a step back to look at the larger situation. Sure, impacts will probably continue and even worsen for some areas. And due to a historic pulse of water heading downstream, the hammered city of Houston is far from out of the woods.

But as with Sandy and so many other freakish strong storms in a present climate that has warmed by around 1.2 C above pre-industrial values, we would be remiss if we didn’t discuss the climate change related factors that gave Harvey more fuel, that helped it to rapidly intensify, that worsened its flooding — both from rains and from storm surge, and that may have helped to produce a still pocket in the upper level winds that allowed it to stick around for so long.

Warmer Ocean Surfaces Mean More Rapidly Intensifying Storms, Higher Peak Intensity of Worst Storms

Hurricanes like Harvey cannot readily form in cool waters below a range between 70 and 75 degrees Fahrenheit. Ideally, the storms require ocean surface temperatures warmer than 80 degrees (F). And the more heat that’s available at the ocean surface, the more energy that’s available for a storm when it does form.

This energy comes in the form of atmospheric lift. In other words, air rises off the water more vigorously as water temperature rises. This lifting energy is called convection. And the more that’s available, the more powerful storms can ultimately become.

(Sea surface temperatures were between 1 and 2 degrees Celsius above average as Harvey approached Texas. Human-forced climate change is causing the oceans to warm. This, in turn, provides more fuel for hurricanes like Harvey — helping them to rapidly intensify and pushing their peak strength higher. Image source: NOAA.)

According to Dr Michael Mann, Ocean surfaces in the Gulf of Mexico are fully 1 to 1.5 degrees Celsius warmer, on average, than they were just 30 years ago. This warming provides more energy for storms that do form. And this, in turn, raises the top potential intensity of storms.

Some scientists, like Dr. James Hansen, refer to this prevalence of worsening extremity as loading the climate dice. If, in the past, we were rolling with a die six with a 1 representing the lowest storm intensity and a 6 representing the highest, we’re now rolling with something like a die six +1. The result is that the strongest storms are stronger and the absolutely strongest storms have an ability to achieve previously unattainable strengths due to the fact that there’s a lot more energy there to kick them into a higher state.

Increased potential peak storm intensity as a climate change factor does not necessarily result in more tropical storms forming overall. That part of the science on hurricanes is highly uncertain. But that heat engine in the form of warmer surface waters is available for the storms that do form to tap. And that can make them a lot stronger and more damaging than they otherwise would have been.

(Loading the climate dice — changes in frequency of cold and warm temperatures also has an impact on heatwaves, droughts, wildfires, storm intensity, and heavy precipitation events. Image source: NASA.)

As Harvey approached land, it tapped the energy of this much warmer than normal ocean surface. And that energy caused the storm to rapidly strengthen — first from a minimal tropical storm to a Hurricane, and then from a minimal hurricane to a Category 4 monster. Meteorologists tend to call such periods of rapid intensification — bombification. This term comes from minimum pressures that rapidly drop in swiftly strengthening storms — seeming to bomb out. And due to warming, the science indicates that rapid strengthening is also more likely. With some models pointing toward a 10-20 fold increase in the frequency of rapidly intensifying storms by the end of this Century if human forced warming of ocean surfaces continues.

Warmer Atmosphere Means Heavier Rainfall

Related to a warming of the ocean surface (and land surfaces as well) is the basic scientific fact that such warming causes the amount of water vapor in the atmosphere to increase. In total, with each 1 degree Celsius of warming near the Earth’s surface, the atmosphere ends up holding about 6-7 percent more moisture. The properties of this warming-driven increase in atmospheric moisture are described by the scientifically proven Clausius–Clapeyron relation which defines, in meteorology, how atmospheric water vapor content is driven by various factors, including temperature.

If we dig just a little bit further into our understanding of how this scientific driver impacts the atmosphere in a warmer world, we find that not only does the moisture content of a warmer atmosphere increase, but both the rates of evaporation and precipitation increase.


(Global warming has brought with it a sharp increase in the number of record-breaking daily rainfall events. This is due to the fact that a warmer world holds more storm-fueling moisture in its atmosphere. This warmer, wetter atmosphere increased the peak potential rainfall from Harvey enabling it to smash records for rainfall rates and precipitation totals. Image source: Increased Record-Breaking Precipitation Events Under Global Warming.)

It is here that we return to the loaded climate dice mentioned above. If, as we find today, the Earth is about 1.2 degrees Celsius warmer than in the past, then the atmosphere holds more moisture. About 7-8 percent more. And since there’s more heat, evaporation is more intense where it does happen. This loads the climate dice for more intense droughts. But since what goes up in the form of evaporation results in a heavier load of moisture in the higher clouds and in the storms that do form, the rains that follow will also tend to be more intense. This loads the dice for more severe rainfall events. And we have a very clear scientific observation that the most extreme rainstorms are becoming much more intense overall (see above graphic).

For Harvey, this meant that more moisture was available to provide the record-setting rainfall amounts coming from that system. Peak rainfall totals from the storm are now at nearly 52 inches. This is the most rainfall ever to occur in Texas from a tropical system in our records. A measure that may also break the all-time U.S. record for rainfall from a tropical storm. And Harvey was enabled to produce such high rainfall amounts by a warmer atmosphere.

Harvey a Brown Ocean Cyclone?

Increasing rates of evaporation and precipitation had one obvious effect in Harvey — they increased the potential severity of rains coming from this kind of storm. But they also increase the ability of storms like Harvey to maintain strength or even intensify over land. If, for example, a storm like Harvey dumps a very heavy load of rainfall over land and if the evaporation from these recent rains has increased in a warming world, then storms like Harvey can tend to draw strength back from what amounts to a small ocean on land.

A recent NASA scientific paper on this issue describes a Brown Ocean effect. The 2013 NASA paper noted:

Before making landfall, tropical storms gather power from the warm waters of the ocean. Storms in the newly defined category derive their energy instead from the evaporation of abundant soil moisture – a phenomenon that Andersen and Shepherd call the “brown ocean.”

…The research also points to possible implications for storms’ response to climate change. “As dry areas get drier and wet areas get wetter, are you priming the soil to get more frequent inland tropical cyclone intensification?” asked Shepherd.

In essence, cyclones are better able to maintain strength or even re-intensify over wet areas of land in a warmer world due to increasing levels of evaporation and it was Harvey’s ability to maintain tropical storm strength over land for up to three days that helped to enable it to keep dropping such heavy volumes of rain.

Higher Seas Mean Worse Storm Surge Flooding

A warmer climate also brings with it the melt of continental glaciers and the thermal expansion of ocean waters. As glaciers flood into the world’s oceans, they rise. And since fossil fuel burning began at the start of the industrial age this related warming of the Earth and melting of glaciers has caused the oceans around the world to rise by more than 20 centimeters globally.

(Global warming increases the base ocean level which, in turn, worsens storm surge flooding. Harvey’s storm surge came in on this higher ramp. Image source: Sea Level Rise Science.)

Such higher seas alone are causing some coastal settlements to flood even on sunny days. But when storms like Harvey come roaring ashore, they do so on a higher overall launching pad. And this produces a multiplier effect for storm surge damages. A multiplier that would not have been there if the world hadn’t warmed.

Polar Warming Contributes to Blocking Patterns That Make Weather Stick Around in One Place Longer

Another climate change related factor that contributed to Harvey’s danger was its persist hovering over the same region. Harvey would not have been as damaging for Texas and the Gulf if it hadn’t hung over East Texas for more than five days. But here, again, we find that climate change related factors appear to be contributing to the increased lingering of various extreme weather producing systems.

To understand how, we need to look at the upper level atmospheric circulation pattern that moves weather systems from place-to-place. In other words — the Jet Stream. Climate change influences the Jet Stream by generating more warming at the poles than near the Equator. This in turn, according to the research of scientists like Dr Jennifer Francis, changes atmospheric slope. Warmer poles, in other words, create a taller atmosphere at the poles relative to the Equator.

(A high amplitude ridge-trough pattern helped to create a stagnant upper air slot in which Harvey stalled. This voiding of upper level steering currents enabled Harvey’s persistence. Some scientists are pointing toward increasing prevalence of these kinds of high amplitude ridges and troughs related to polar warming warming which is an upshot of global warming. Image source: Earth Nullschool.)

Since atmospheric slope and temperature differences between the poles and Equator drive the speed of the Jet Stream, warmer poles cause the Jet Stream to slow down and meander. This generates big ridges and deep troughs. It also appears to assist the generation of large blocking high pressure systems. And all of these features can tend to cause weather patterns to get stuck.

This year, a persistent trough in the Eastern U.S. has generated a consistent stormy pattern and aided in the firing of powerful thunderstorms that produced record rains over places like Kansas City, Missouri. In the West, a persistent ridge has produced record heat and very extreme wildfires while aiding the formation of a very intense flash drought in Montana and the Dakotas. Harvey got stuck in a stagnant pocket between these two relatively fixed weather patterns. A climate change related feature that may have increased the duration of Harvey and facilitated its record rains falling over such a long period.

Other Factors — Interaction With The Eastern Trough

Finally, we can state that Harvey’s interaction with the very deep eastern trough also helped to fuel it. The trough provided a moisture and instability kick to Harvey as it moved over Texas — helping to wring out tropical moisture over the Lone Star State. And if we accept the fact that polar warming contributed to the depth of this eastern trough by slowing down the Jet Stream, then its interaction with Harvey was also a climate change related factor.

Qualifying This Discussion

What can be said with certainty is that climate change did not cause the hurricane. That hurricanes do happen in a normal climate. But this is the same same thing as saying that home runs happen in both middle school and major league baseball. It’s all baseball, but the factors from one to the other have qualitatively changed in an obvious fashion. The same thing happens to weather in a warming world. And it is due to the changes in these underlying factors that we can say without a doubt that climate change made Harvey worse.

What we can also say is that our certainty of all these various climate change related factors involved varies. For example, we can say with very high certainty that global warming is worsening rainfall extremes and that sea level rise is worsening storm surges. We can say with a good level of confidence that the peak intensity of the worst storms is also increasing and that bombification is more likely. And we can say with moderate confidence that climate change is altering atmospheric circulation patterns (an issue that is still under considerable debate).

But the varying degrees of certainty with regards to these aspects do not change basic facts. Your climate is your weather averaged over 30 years. And if the world warms, both your climate and your weather change.