Hurricanes & Climate Change

Two-hundred years ago French mathematician Joseph Fourier first proposed that gases in our atmosphere create a "greenhouse effect" that warms the earth.  Several decades later Irish physicist John Tyndall proved that increasing the amount of greenhouse gases in the atmosphere would warm the earth -- namely, increasing carbon dioxide (CO2) would lead to rising temperatures.

Since the 1950s, measurements show that the amount of CO2 in the atmosphere has been rising steadily; measuring the amount of carbon dioxide trapped in ancient ice has told us that CO2 levels are higher today than they have been in at least 650,000 years

The science is settled: climate change is real, it's happening now, it's caused by humans, and it's fixable.

So, will climate change have an impact on hurricanes and hurricane season in the future?  A recent overview of current research results conducted by NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) says yes, with greater than 2-to-1 odds.

"It is likely that greenhouse warming will cause hurricanes in the coming century to be more intense globally and have higher rainfall rates than present-day hurricanes."

What about this hurricane season; has global warming played a role?  That's a question that scientists are still wrestling with, though one thing is certain: Hurricane Harvey and Hurricane Irma weren't caused by global warming.  Both storms hit during the historical peak of hurricane season.  Still, the 2017 Atlantic hurricane season has already delivered a lot of firsts:

  • Tropical Storm Arlene hit in April 2017 and was 1 of only 2 named tropical storms on record in April, as well as the northernmost on record for that time of year
  • Hurricane Harvey's five-day deluge resulted in the highest rainfall on record from a single storm in the continental U.S.
  • Hurricane Harvey will likely end up being the costliest hurricane in U.S. history
  • Hurricane Irma reached Category 5 strength farther east than any storm on record

Looking at how hurricanes form, the physics of climate change, and weather conditions this summer, many see the fingerprints of global warming on this list of firsts.

Hurricanes won't form over ocean water that's cooler than 80°F; the warmer the water, the stronger the storm can become.

In a world warmed by climate change we know that the world's oceans will also be warmer. In fact we've seen year-round ocean temperatures rise on average over the last 100 years.  In theory, these warmer temperatures should allow hurricanes to form earlier in the year, and at higher latitudes than before.  But, we can't yet attribute the strength of Arlene, Harvey, or Irma to climate change for certain.

This summer, NOAA found that ocean temperatures in eastern tropical Atlantic were 1°F to 3°F above the 30-year average.  In theory, these above-average temperatures should allow for stronger storms to form and to persist.  But, we can't yet attribute the strength of Arlene, Harvey, or Irma to climate change for certain.

In the open ocean, water temperatures control the temperature of the overlying air above; so, warmer water creates warmer air.  A warming atmosphere causes more evaporation, meaning more water is available for precipitation. For every 1°F increase in temperature, the atmosphere can hold around 4% more water vapor; but, we can't yet attribute the rainfall totals from Harvey or Irma to climate change for certain.

The record-breaking rain that fell over Texas during Hurricane Harvey (51.88" near Mont Belvieu, TX) was due in part to the system lingering over the region.  Multiple researchers, including Michael Mann in this March 2017 publication, have linked "blocky" weather patterns and an increase in stalled weather systems to climate change.  While the historic stall of Harvey in Texas appears to fit this mold, we can't yet attribute that outcome to climate change for certain.

Sea-level rise caused by climate change is already occurring, and a January 2017 NOAA report states that global mean sea levels have risen 8 to 9 inches since 1880, with about 3 inches of rise occurring since 1993.  These rises have already affected coastal areas of the U.S. in a noticeable way, and modeling completed for one study has shown "that, by mid-century, some locations may experience high water levels annually that would qualify today as 'century' (i.e., having a chance of occurrence of 1% annually) extremes."  Of course the storm surge for any landfalling hurricane could be made worse by climate change effects but, at this time, surge levels from Harvey and Irma can't yet be attributed to climate change.

One characteristic of climate change that could work against hurricane formation is wind shear, or the vertical change of wind speed and direction.  Several well-regarded climate scientists (such as Vecchi and Soden, and the National Hurricane Center's Chris Landsea) have published studies that show an increase in vertical wind shear is likely in a warming climate.  Hurricane formation and intensity is inhibited by increasing wind shear.

Though there will likely be fewer Atlantic hurricanes overall by 2100, wind speeds for the ones that do form will be about 4 percent stronger for every 1°C increase in sea surface temperature. Graph credit: Tom Knutson, NOAA GFDL

The bottom line is that more study is needed to fully determine how climate change will influence individual hurricane seasons, or to attribute specific hurricane outcomes to climate change.  However, there are strong indications at this point that climate change is likely to cause more Category 4 and 5 storms.