Thursday, April 6, 2017

NOAA Admits No Link Between Hurricanes and Warming

But they're still holding out hope: Global Warming and Hurricanes – NOAA says no measurable effect yet
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?

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.

So let's look at these models:

Figure 1: Two different statistical models of Atlantic hurricane activity vs sea surface temperature (SST). The upper panel statistically models hurricane activity based on “local” tropical Atlantic SST, while the bottom panel statistically models hurricane activity based on tropical Atlantic SST relative to SST averaged over the remainder of the tropics.Both comparisons with historical data and future projections using this approach are shown. See Vecchi et al. 2008 for details

The first thing to note is that the model are very noisy, the collection of models have an extremely wide spread and even at the beginning, don't match the actual data (in black) very well. I presume the rise in the power dissipation in the top figure is based on model temperature rises, the very same models that have rather consistently over the predicted rate of rises in temperature by a factor of 3 or so. Second, if you base the models on relative sea surface temperature, a reasonable assumption, as it is the difference in temperatures that mostly drives the weather, a much more conservative result is scene, one in which the change over upcoming century is well within the model disagreements, and there is very little effect of warming on tropical storms. So, even if it does warm, hurricane activity is not likely to increase much, if at all.

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