Hurricanes and Climate Change

This post is intended to provide an overview of climate change and tropical cyclones. I’m going to start this post with some commentary to set the scene:

The detection and attribution of the possible effects of anthropogenic climate change on tropical cyclones is one of the most controversial topics in present-day science. The increase in tropical cyclone numbers in the Atlantic since the mid 1990s, combined with the devastating impacts of individual hurricanes such as Katrina in 2005, has let to an urgent examination of trends in the available tropical cyclone data to see if these can be explained by man’s effect on the climate.

Walsh et al (2009)

Observations through early October 2006 show that we have so far experienced an average Atlantic basin hurricane season. August had substantially below-average activity (only 45% of average) while September had above-average activity (about 140% of average). US landfall has been well below average. No hurricanes have made landfall along the US coastline this year. This has occurred in only 18 percent of the hurricane seasons since 1945. In an average year about 90 percent of the seasonal average NTC of 100 occurs by 11 October. 

Gray (2006)

El Niño years typically have the following tropical Atlantic conditions:

1.     stronger than normal 200 mb (~12 km) zonal winds (positive U),

2.     dryer middle tropospheric moisture conditions (negative q – specific humidity),

3.     somewhat higher than average sea level pressure anomalies (positive SLPA),

4.     somewhat higher than average sea surface temperature anomalies (positive SSTA).

Gray (2006)

Large amplitude fluctuations in the frequency and intensity of tropical cyclones greatly complicate both the detection of long-term trends and their attribution to rising levels of atmospheric greenhouse gases. Trend detection is further impeded by substantial limitations in the availability and quality of global historical records of tropical cyclones.

Knutson et al (2010)

But what is the situation now?

The IPCC (2013) provide a stark overview:

Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.

Of critical importance for tropical cyclones:

Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010 (see Figure SPM.3), and it likely warmed between the 1870s and 1971.

And vitally:

Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system.

So what does all this mean for the future of tropical cyclones?

The ambiguity of scientific literature investigating linkages between climate change and tropical cyclones is clear. Few papers agree on projected characteristics of storm systems in terms of their frequency and intensity. Much of the research suggests that regional changes will be of chief importance and as a result there is an absence of a generally agreed framework or theory regarding the impact of anthropogenic climate change on hurricanes.

If anthropogenic climate change causes an increase in hurricane activity, shouldn't we see positive trends over, say, the last 100 years? 

When accounting for missing data over that time period, statistical tests reveal that relative to the variability in the dataset, a correlation is not significantly distinguishable form zero. Hurricane numbers were high enough in the 1960s for a couple of decades that there is no significant positive trend from that period in time. Landfalling hurricanes even show a slight negative trend through time. More interesting, perhaps, is the apparent increase in the number of major hurricanes over this period - where major hurricanes are considered Category 4/5. However, the storms that do occur in the warmer climate simulation are more intense on average than those in the control (present day) simulation. 

The 'facts' are:

• Globally there has been no increase in tropical cyclone frequency over the past few decades (e.g. Webster et al, 2005; Elsner and Kocher, 2000).

• Storm frequency has not tracked recent tropical climate trends (Pielke Jr et al, 2005).

• The metrics of tropical cyclone intensity are varied and may be limited by data.

• While observations of tropical and subtropical sea surface temperature have shown an overall increase of about 0.2°C over the past ~50 years, there is only weak evidence of a systematic increase in potential intensity (Bister and Emanuel,2002; Free et al, 2004).

• Other authors suggest the intensity of Atlantic tropical cyclones is rising dramatically.

Elsner (2006) presents an interesting report on two competing hypotheses surrounding causes of increasing Atlantic hurricane activity. One hypothesis, known as the climate change hypothesis, suggests increases in greenhouse gases and their associated changes in eradicative forcing can cause SST in the Atlantic to rise through higher global temperatures. On the other hand, the Atlantic Multidecadal Oscillation (AMO) hypothesis suggests that natural ocean circulation changes in the Atlantic drive hurricane season SST, leading to changes in hurricane frequency and global temperatures. The common agenda in both competing theories is that regional SST is crucial in the genesis of tropical cyclones. The main discussion point is the interconnectivity between global temperatures and SST in the Atlantic. The jury is still out and the challenge remains to find the detailed causal mechanism between climatic change and tropical cyclone activity.

One of the most important pieces of research on the subject has come from Knutson et al (2008) in their letter to Nature. The authors present the results of a study investigating the changes in large-scale climate through the 21^st century using an ensemble of global climate models and describe that Atlantic hurricane frequencies reduce. The results ‘do not support the notion of large increasing trends in either tropical storm or hurricane frequency driven by increases in atmospheric greenhouse-gas concentrations’.