Storms and temperature trends

July 31, 2007 at 3:53 pm | Posted in Climate change, Cyclones, Holland, Hurricanes, Royal Society, Storms, Temperatures, UCAR, Webster | Leave a comment

A new climate study indicates that hurricanes and tropical storms became more frequent in the Atlantic Ocean during three distinct periods over the last century, as shown in this graphic.The first part of the 20th century (in white) was relatively quiet, with an annual average of 6 observed hurricanes and tropical storms.

The annual average increased to 10 after 1930, and then reached 15 from 1995 to 2005 (in darkest shading).

This graphic shows both the total number each year (blue line) and the nine-year running average, calculated from four years back through four years ahead of a given year. Called a running mean, this method smoothes out year-to-year variability to reveal the long-term trend.

The new research associates the increasing storms with rising sea-surface temperatures. Illustration by Steve Deyo, ©UCAR.

Frequency of Atlantic Storms chart to illustrate findings of Holland & Webster paper

The above quote and illustration are taken from the University Corporation for Atmospheric Research (UCAR) news release and links to the study are here.

At a very simple broadbrush level, there are several ways to use a chart like this in presenting observed trends to students and other interested parties, without getting into too much detail:

  • One is to discuss the fact that we have more accurate ways of measuring events nowadays than we did in the past, so we need to assess how much of the increase in the number of Atlantic storms is due to improved detection, measurement, recording and analysis of such storms. Next, we can look into whether the authors, Holland and Webster, took historical uncertainties of measurement and developments in analysis into account (they did) in producing this report.  We could plot 1944 (start of aircraft data) and 1970 (start of satellite data) on the chart to see if the marked step-ups in average numbers of storms occur on or around these years (they don’t; 1930 and 1995 are the years when step changes in activity appeared to occur).  For the record, this is what the news release has to say on the topic of accuracy and its significance:

The study also finds that enhanced observations in recent decades cannot account for all of the increase. To observe storms in the Atlantic more systematically, meteorologists began relying on data from aircraft flights in 1944 and satellites about 1970. The distinct transitions in hurricane activity noted by Holland and Webster occurred around both 1930 and 1995.

The Holland & Webster study itself provides further detail:

We have given careful consideration to the quality of the data and particularly to changing observing system and analysis practices. We have noted with some concern the contradictory conclusions reached in a number of papers that, on the one hand, describe the data as being of high quality sufficient to determine natural variability in hurricane characteristics ( Landsea et al. 1999; Goldenberg et al. 2001; Owens & Landsea 2003) but, on the other hand, of insufficient quality to determine trends that are demonstrably of similar magnitude ( Landsea et al. 2006).  We concur with Emanuel (2005), Webster et al. (2005) and Hoyos et al.  (2006) that the trends since 1900 shown in figure 1 are substantial and the related linear trend is significant at the 95% level (R. Smith 2007, personal communication). We are of the strong and considered opinion that data errors cannot explain the sharp high-amplitude transitions between the climatic regimes in the NATL, each with an increase of approximately 50% in cyclone and hurricane numbers, and the close relationship of these regime changes with SSTs.

  • Another is to compare the different trend lines for annual totals and nine-year running mean throughout the century. Then, we can consider whether the running mean helps people appreciate the long-term trend for this century more easily than the annual totals, which vary widely from year-to-year. (It is often easier to grasp a fundamental truth or change in progress by standing back from the coalface and looking again from a distant viewpoint. Grandparents who see their grandchildren infrequently always notice the difference in height and maturity more sharply than parents who live with their own children day after day. Similarly, slow-heating a live frog to boiling point, compared with dropping one suddenly into boiling water, is another favourite image that brings the message home to kids … and not just because it was used in AIT.)

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