Human Influence On Climate

“Human influence” frequently is cited as driving changes in earth’s climate and is believed by many to have increased events of extreme precipitation in the United States, melted Arctic sea ice, and triggered European flooding. However, in the last couple of weeks, a remarkable string of research has been published that indicates Mother Nature’s strength has yet to be exceeded by humans.
      U.S. Precipitation. Climatologists led by the Illinois State Water Survey’s Ken Kunkel reanalyzed the United States’ precipitation history using a newly released dataset from the National Climatic Data Center (NCDC) that contains a significant amount of data that extends back to 1895. Similar, earlier research is limited to data beginning a couple of decades later and which is more sparse (e.g., Karl and Knight, 1998).
      Karl’s and Knight’s research relied on data from 1910 forward and reported a disproportionate increase in extreme one-day precipitation. The Intergovernmental Panel on Climate Change (IPCC) later cited their report as evidence that humans already alter climate in ways that have negative impacts. This is significant because the raison d’etre for the Framework Convention on Climate Change that spawned the IPCC is to prevent dangerous human influence on climate. (We will note as an aside that the IPCC was selective in its use of the report in stating, for example, that more extreme rainfall is bad while ignoring another result by Karl and Knight that recognizes more total rainfall can be good).
      Kunkel et al used the more extensive NCDC data to evaluate whether the trends toward more extreme one-day rainfall reported by Karl and Knight were a continuation of something that began at an earlier date. What they found was very nearly the opposite. At the beginning of the record, the frequency of extreme one-day precipitation events is about the same as at its end! In fact, there was a big dip between the two endpoints (see Figure 1). This means that the increased frequency of extreme rainfall during the latter half of the 20th century is no more unusual than the decrease in the frequency of extreme events in its first half. In other words, extreme U.S. one-day precipitation events occur at about the same frequency now (during a period of presumed human influence) as they did in the late 19th century (a time relatively free of large-scale human interferance).
      How, then, can our current climate be unusual in comparison with “natural” climate? Here’s how Kunkel’s team addresses the issue:

For one-day duration events, recent increases in frequencies are of comparable magnitudes to frequencies around the turn of the [20th] century, suggesting the possibility that natural variability could be an important contributor to recent increases. The greater length of record used in this analysis, compared to many previous studies, establishes an important context for understanding recent changes in the U.S. For example, inspection of time series of extremes based on just the last 50-70 years lead to quite different qualitative conclusions than those based on the 107-yr record used here.

Figure 1. A measure of the frequency of occurrence of the extreme one-day precipitation shows that across a range of “extreme” definitions, extreme one-day precipitation events occurred as often in the late 19th century as they did in the late 20th. (Taken from Kunkel et al., 2003). Arctic Sea Ice. German researchers Cornelia Köberle and Rödiger Gerdes research results concern their attempt to enhance data on Arctic sea ice by increasing its temporal and spatial resolution. Good coverage did not begin until the late 1970s with the advent of satellite observations, so they constructed an Arctic sea ice model based upon atmospheric observations (temperature, pressure, wind speed and direction, etc.) available since the late-1940s in consolidated form from the National Center for Environmental Prediction (NCEP).       Köberle and Gerdes believe their model successfully matches observations of various aspects of Arctic sea ice (its thickness, extent and transport) and used the model to simulate ice conditions between 1948 and 1997. Figure 2 depicts their results for Arctic ice volume.       They find a large decline in ice volume from the late-1960s to the late-1990s. This is consistent with observations that are the basis for proclamations of a near-future demise of ice in the Arctic as a consequence of human-induced global warming. But they also find there to have been a huge run-up in Arctic ice volume in the decades prior to the late-1960s. Taken together, this means there was no trend in the volume of Arctic sea ice from 1948 to 1997 (at least as reproduced Köberle’s and Gerdes’ model). They conclude: These results make connecting “global warming” to Arctic ice thinning very difficult for two reasons. First, large decadal and longer-term variability masks any trend. Restricted time series…produce trends that are more or less arbitrary….Second, the wind stress strongly effects the long-term development of ice volume. A long-term change in wind stress over the Arctic, possibly by an increase in the number of atmospheric circulation states that favor ice export, would effect the ice volume in a similar manner as a temperature increase. Figure 2. Simulated Arctic ice volume in the model by Köberle and Gerdes. Notice how the volume of Arctic ice was as low (or lower than) in the early 1950s as in the late 1990. (Taken from Köberle and Gerdes, 2003) European Flooding. It was a coincidence but, during the time of European flooding last summer, German scientist Manfred Mudelsee was leading an ongoing research project to establish an historical record of flood events on two major central European rivers, the Elbe and the Oder.       Recently, Mudelsee et al concluded their effort and determined (1) there to have been no long-term trend in major summertime flooding events along these two rivers during the 20th century (even as the frequency of winter flood events decreased — see Figure 3) and (2) last year’s flooding along the Elbe, while large and catastrophic by recent standards, has a “return interval” of about once every 168 years. There have been similar-sized events in the past; a person only had to look for them, as Mudelsee demonstrates. As a consequence, there is no justification for invoking global warming as the culprit in 2002’s inundation. Mudelsee introduces his research results this way: Extreme river floods have been a substantial natural hazard in Europe over the past centuries, and radiative effects of recent anthropogenic changes in atmospheric composition are expected to cause climate changes, especially enhancement of the hydrological cycle, leading to an increased flood risk. and concludes, Although extreme flood with return periods of 100 years and more occurred in central Europe in July 1997 (Oder) and August 2002 (Elbe), there is no evidence from the observations for recent upward trends in their occurrence rate.           German flood-protection expert Otto Malek tells Nature magazine, “The real problem is that there are too many people building houses, and amassing material assets, in areas known to be in danger of flooding.”

Figure 3. Occurrence of flooding of central Europe’s two major rivers, the Elbe and the Oder. Notice there are no signs of increases in recent flooding. In most cases 20th-century trends are downward or toward less flooding. (Taken from Mudelsee et al., 2003).

As researchers increasingly turn their efforts toward climate and climate change, it is becoming increasingly obvious that our current climate isn’t all that interesting from an historical perspective. If one looks objectively at the big, complicated picture of the world’s climate Mother Nature has placed us into, our present circumstance reduces us to one of those “Where’s Waldo” icons where what makes us unique becomes pretty difficult to discern in the midst of everything else in the background.

References:

Karl, T.R., and R.W. Knight, 1998. Secular trends of precipitation amount, frequency, and intensity in the United States. Bulletin of the American Meteorological Society, 79, 231-241.

Köberle, C., and R. Gerdes, 2003. Mechanisms determining the variability of Arctic sea ice conditions and export. Journal of Climate, 16, 2843-2858.

Kunkel, K.E., et al., 2003. Temporal variations of extreme precipitation events in the United States: 1895-2000. Geophysical Research Letters, 30, doi:10.1029/2003GL018052.

Mudelsee, M., et al., 2003. No upward trends in the occurrence of extreme floods in central Europe. Nature, 425, 166-169.

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