What Hurricane Harvey tells us about climate change

The heartbreaking consequences of Hurricane Harvey’s landfall in Texas and Louisiana over the past week have led many public figures to comment regarding the potential connection between hurricanes and global climate change. With Hurricane Irma bearing down on the Caribbean and Florida, this question will likely get another bump in the news cycle.

What does the science tell us about this connection? In 2014, the White House released the latest update of the congressionally mandated National Climate Assessment (NCA), a report produced by the relevant scientific agencies every few years. The NCA made the following statement:

The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.

That remains a good summary as far as hurricanes go, but there is a more fundamental point that I think decision-makers should be focused on as they consider how to direct investments to enhance our resilience to climate change: Precipitation extremes are intensifying and will continue to do so as the climate warms.

Consider the following:

  • In April 2014, southern Alabama and the Florida Panhandle experienced a historic rainstorm that set local records for daily and hourly rainfall totals. In Pensacola, nearly 6 inches of rain fell in an hour and more than two feet of rain fell over two days, causing catastrophic flash flooding.
  • In October 2015, many areas of Alabama, Louisiana, and Texas experienced extreme rainfall and flash flooding. The greatest totals were south of Dallas, where more than 20 inches of rain flooded highways and derailed a freight train. Houston received around 10 inches and experienced flash flooding.
  • In March 2016, Louisiana experienced historic flooding and areas of Arkansas, Missouri, Oklahoma, Tennessee, and Texas experienced extreme rainfall and flash flooding. More than 20 inches of rain fell in Monroe, Louisiana, with one reporting station recording nearly 27 inches over three days.

Now we have Hurricane Harvey.

These events are a small selection of a large number of major flood events to strike Arkansas, Louisiana, Oklahoma, and Texas recently. Gulf states have seen historic flooding disasters from extreme precipitation every year for at least four years running. Many of these events were associated with hurricanes (or tropical storms in general). All of them required an enormous source of atmospheric moisture to generate such extreme rainfall totals in a matter of hours to days. 

That moisture source is no mystery: It is the warm tropical waters of the North Atlantic (principally the Gulf of Mexico) and eastern Pacific oceans. These bodies of water have been warming over recent decades and are evaporating more and more moisture into the atmosphere along the Gulf and Atlantic Coasts. The atmosphere is also warming, and warmer air holds more water vapor. As the climate warms, therefore, more moisture becomes available to supply rainfall.

This fact is basic physics and there isn’t any real uncertainty about it. Moreover, it is well understood that the oceans and atmosphere are warming as a direct result of manmade greenhouse gas emissions. (Without those emissions, the climate system would actually be cooling slightly).

The consequences are not limited to the Gulf Coast. The National Climate Asessment chart below shows the percent increase in the amount of rainfall associated with the heaviest 1 percent of downpours in regions of the United States from 1958 to 2012.

So what does Hurricane Harvey tell us about climate change? It confirms that our risk is rising as the climate warms. Harvey also teaches us about our vulnerabilities and adaptation needs.

As Dan Huber and I have explained, individual weather events are unpredictable, but our overall risk from changing weather patterns is predictable. Reducing our climate-warming greenhouse gas emissions will limit how much we ultimately ratchet up that risk. However, since the climate is already changing and the risk of damages is rising, we also need to adapt to the changes that are in the pipeline. Both reducing emissions and adapting to unavoidable change are essential to managing the risk

Jay Gulledge, Ph.D., is a Senior Adviser to C2ES

Countries should assess climate risk the way they assess other security risks

National security leaders deal with deep uncertainty on a daily basis about everything from North Korea’s ability to produce a nuclear weapon to the location and timing of the next terrorist attack by non-state actors such as ISIS and al-Qaida. Security decision-makers don’t use uncertainty as an excuse to ignore security threats.

Borrowing a page from security analysts, a new report out today by renowned climate experts and high-level government advisors from China, India, the United Kingdom and the United States assesses the risks of climate change in the context of national and international security.

Accounting for scientific uncertainty in a dangerously warming world

Most people at some point develop a “Plan B” – in case their first choice of college doesn’t accept them, or it rains on the day of their planned outdoor party, or the deal for the house they wanted falls apart. The same principle applies for more dire situations, such as a city having plans in hand for an orderly evacuation in case of a large-scale disaster. We hope such an event will never happen, but the mayor had better be prepared in case it does.

In a commentary today in the scientific journal Nature Climate Change, three colleagues and I discuss the need for a “Plan B” for climate change: How will we cope with increasingly severe climate impacts if we are unsuccessful in limiting global warming to a chosen target?

In the 2009 Copenhagen climate accord, countries set a goal of limiting global warming to below 2 °C (3.6 °F) above the average global temperature of pre-industrial times. However, given that the planet has already warmed by 0.8 °C, additional warming is already locked into the system, and global greenhouse gas emissions continue to rise, this “Plan A” has become increasingly difficult and may become impossible to achieve if widespread emissions reductions do not begin within this decade. A maximum warming target is a necessary goal of climate policy, but what if our efforts fall short?

Some voices in the environmental community will feel that asking this question is ceding failure, but I disagree. Instead, it means admitting that we can’t perfectly foresee the future and that we need to be prepared for surprises. This is called risk management and everyone from parents, to mayors, to companies, to the U.S. military uses risk management every day to cope with uncertainty.

Two More Lows for Arctic Sea Ice

The loss of Arctic sea ice is progressing more rapidly and clearly than just about any other indicator of global climate change. As I’ve discussed previously, the minimum summer sea ice extent (i.e. the two-dimensional area of the floating ice cap) set new record lows in 2002, 2005 and 2007. Similarly, the total volume of sea ice set record lows in 2007, 2010 and 2011. For the first time since 2007, both the sea ice extent and volume have set new record lows in the same year (see figures). And what’s more, they did it with weeks remaining in the melt season, which usually ends in mid-September. So the records have been broken this year, but we don’t know yet just how low the extent and volume will go.


Senate gets back to climate science

The Senate Environment and Public Works Committee holds a hearing tomorrow called “Update on the Latest Climate Change Science and Local Adaptation Measures.” This is the first Senate hearing focused directly on climate science in the 112th Congress, and we hope it won’t be the last. Climate change is happening, the news from peer-reviewed science is increasingly daunting, and the public needs to hear what credible scientists are learning about the risks and potential solutions.