Two estimates of percent change in hurricane power dissipation index (PDI), projected to 2100. Graphic: NOAA

30 August 2017 (NOAA) – Two frequently asked questions on global warming and hurricanes are the following:

Have humans already caused a detectable increase in Atlantic hurricane activity or global tropical cyclone activity?

What changes in hurricane activity are expected for the late 21st century, given the pronounced global warming scenarios from current IPCC models?

1. Summary Statement

In this review, we address these questions in the context of published research findings. We will first present the main conclusions and then follow with some background discussion of the research that leads to these conclusions. The main conclusions are:

  • It is premature to conclude that human activities–and particularly greenhouse gas emissions that cause global warming–have already had a detectable impact on Atlantic hurricane or global tropical cyclone activity. That said, human activities may have already caused changes that are not yet detectable due to the small magnitude of the changes or observational limitations, or are not yet confidently modeled (e.g., aerosol effects on regional climate).
  • Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones globally to be more intense on average (by 2 to 11% according to model projections for an IPCC A1B scenario). This change would imply an even larger percentage increase in the destructive potential per storm, assuming no reduction in storm size.
  • There are better than even odds that anthropogenic warming over the next century will lead to an increase in the occurrence of very intense tropical cyclone in some basins–an increase that would be substantially larger in percentage terms than the 2-11% increase in the average storm intensity. This increase in intense storm occurrence is projected despite a likely decrease (or little change) in the global numbers of all tropical cyclones.
  • Anthropogenic warming by the end of the 21st century will likely cause tropical cyclones to have substantially higher rainfall rates than present-day ones, with a model-projected increase of about 10-15% for rainfall rates averaged within about 100 km of the storm center.

2. Global Warming and Atlantic Hurricanes

Atlantic (HURDAT) storms, adjusted for estimated missing storms, 1878-2006. Graphic: NOAA

A. Statistical relationships between SSTs and hurricanes

Observed records of Atlantic hurricane activity show some correlation, on multi-year time-scales, between local tropical Atlantic sea surface temperatures (SSTs) and the Power Dissipation Index (PDI) —see for example Fig. 3 on this EPA Climate Indicators site. PDI is an aggregate measure of Atlantic hurricane activity, combining frequency, intensity, and duration of hurricanes in a single index. Both Atlantic SSTs and PDI have risen sharply since the 1970s, and there is some evidence that PDI levels in recent years are higher than in the previous active Atlantic hurricane era in the 1950s and 60s.

Model-based climate change detection/attribution studies have linked increasing tropical Atlantic SSTs to increasing greenhouse gases, but the link between increasing greenhouse gases and hurricane PDI or frequency has been based on statistical correlations. The statistical linkage of Atlantic hurricane PDI to and Atlantic SST suggests at least the possibility of a large anthropogenic influence on Atlantic hurricanes. If the correlation between tropical Atlantic SSTs and hurricane activity is used to infer future changes in Atlantic hurricane activity, the implications are sobering: the large increases in tropical Atlantic SSTs projected for the late 21st century would imply very substantial increases in hurricane destructive potential–roughly a 300% increase in the PDI by 2100 (Figure 1a).

On the other hand, Swanson (2008) and others have noted that Atlantic hurricane power dissipation is also well-correlated with other SST indices besides tropical Atlantic SST alone, and in particular with indices of Atlantic SST relative to tropical mean SST (e.g., Figure 1b from Vecchi et al. 2008). This is in fact a crucial distinction, because the statistical relationship between Atlantic hurricanes and local Atlantic SST shown in the upper panel of Figure 1 would imply a very large increases in Atlantic hurricane activity (PDI) due to 21st century greenhouse warming, while the statistical relationship between the PDI and the alternative relative SST measure shown in the lower panel of Figure 1 would imply only modest changes of Atlantic hurricane activity (PDI) with greenhouse warming. In the latter case, the alternative relative SST measure in the lower panel does not change very much over the 21st century in global warming projections from climate models, because the warming projected for the tropical Atlantic in the models is not very different from that projected for the tropics as a whole.

A key question then is: Which of the two future Atlantic hurricane scenarios inferred from the statistical relations in Figure 1 is more likely? To try to gain insight on this question, we have first attempted to go beyond the ~50 year historical record of Atlantic hurricanes and SST to examine even longer records of Atlantic tropical storm activity and second to examine dynamical models of Atlantic hurricane activity under global warming conditions. These separate approaches are discussed below.

B. Analysis of century-scale Atlantic tropical storm and hurricane records

Atlantic Basin hurricane counts, 1851-2006. Graphic: NOAA

To gain more insight on this problem, we have attempted to analyze much longer (> 100 yr) records of Atlantic hurricane activity. If greenhouse warming causes a substantial increase in Atlantic hurricane activity, then the century scale increase in tropical Atlantic SSTs since the late 1800s should have produced a long-term rise in measures of Atlantic hurricanes activity.

Existing records of past Atlantic tropical storm or hurricane numbers (1878 to present) in fact do show a pronounced upward trend, which is also correlated with rising SSTs (e.g., see blue curve in Fig. 4 or Vecchi and Knutson 2008). However, the density of reporting ship traffic over the Atlantic was relatively sparse during the early decades of this record, such that if storms from the modern era (post 1965) had hypothetically occurred during those earlier decades, a substantial number would likely not have been directly observed by the ship-based “observing network of opportunity.” We find that, after adjusting for such an estimated number of missing storms, there is a small nominally positive upward trend in tropical storm occurrence from 1878-2006. But statistical tests reveal that this trend is so small, relative to the variability in the series, that it is not significantly distinguishable from zero (Figure 2). In addition, Landsea et al. (2010) note that the rising trend in Atlantic tropical storm counts is almost entirely due to increases in short-duration (<2 day) storms alone. Such short-lived storms were particularly likely to have been overlooked in the earlier parts of the record, as they would have had less opportunity for chance encounters with ship traffic.

If we instead consider Atlantic basin hurricanes, rather than all Atlantic tropical storms, the result is similar: the reported numbers of hurricanes were sufficiently high during the 1860s-1880s that again there is no significant positive trend in numbers beginning from that era (Figure 3, black curve, from CCSP 3.3 (2008)). This is without any adjustment for “missing hurricanes”.

The evidence for an upward trend is even weaker if we look at U.S. landfalling hurricanes, which even show a slight negative trend beginning from 1900 or from the late 1800s (Figure 3, blue curve). Hurricane landfalling frequency is much less common than basin-wide occurrence, meaning that the U.S. landfalling hurricane record, while more reliable than the basin-wide record, suffers from degraded signal-to-noise characteristics for assessing trends.

Normalized tropical Atlantic indices: global mean temperature, tropical Atlantic sea surface temperature (SST), raw and adjusted hurricane counts, and U.S. landfalling hurricanes. Graphic: NOAA

While major hurricanes (Figure 3, red curve) show more evidence of a rising trend from the late 1800s, the major hurricane data are considered even less reliable than the other two records in the early parts of the record. Category 4-5 hurricanes show a pronounced increase since the mid-1940s (Bender et al., 2010) but again, we consider that these data need to be carefully assessed for data inhomogeneity problems before such trends can be accepted as reliable.

The situation for Atlantic hurricane long-term records is summarized in Figure 4. While global mean temperature and tropical Atlantic SSTs show pronounced and statistically significant warming trends (green curves), the U.S. landfalling hurricane record (orange curve) shows no significant increase or decrease. The unadjusted hurricane count record (blue curve) shows a significant increase in Atlantic hurricanes since the early 1900s. However, when adjusted with an estimate of storms that stayed at sea and were likely “missed” in the pre-satellite era, there is no significant increase in Atlantic hurricanes since the late 1800s (red curve). While there have been increases in U.S. landfalling hurricanes and basin-wide hurricane counts since the since the early 1970s, Figure 4 shows that these increases are not representative of the behavior seen in the century long records. In short, the historical Atlantic hurricane record does not provide compelling evidence for a substantial greenhouse warming-induced long-term increase. [more]

Global Warming and Hurricanes

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