Figures Lie and Liars Figure – Under Water

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If you’re new here, you might want to read the previous Figures Lie and Liars Figure posts, a public conversation on climate change between my skeptical aunt and myself.

My aunt’s last question about this EPA slideshow on climate change indicators is about extreme rainfall events.  The slideshow tells us that

In recent years, a higher percentage of precipitation in the United States has come in the form of intense single-day events. Eight of the top 10 years for extreme one-day precipitation events have occurred since 1990. The occurrence of abnormally high annual precipitation totals has also increased.

But my aunt remembers big floods from the 1970s and suggests that those events may cast doubt on intense single-day precipitation events as an indication of climate change.

Single-day events causing floods is talked about as a recent event, but there were similar things in PA in the 70s (huge rains that washed coffins out of the ground)–the trees are still marked down at Knoebels.  It snowed 48″ on Thanksgiving when I was 15–that was a lot of precipitation (1970).

As Brazil, Australia, the Pacific Northwest, South Africa, Denmark, and Sri Lanka are currently struggling with floods and Pakistan is just beginning a long road to recovery after a FIFTH of the country went under last year, I think this is the perfect time to address this issue.  My aunt refers to the flooding from Hurricane Agnes in 1972 in her question. I couldn’t find any records that back up my aunt’s memory of a big Thanksgiving snowstorm in 1970, but perhaps it was a fairly localized event. Or maybe her memory was a little fuzzy and she’s thinking of the smaller (but still record-setting) storm in 1971.

The particular events my aunt refers to don’t actually matter. Her real question is do extreme precipitation events in the past mean that recent and future events have nothing to do with climate change?

Well, yes and no. No individual extreme precipitation event is a sign that climate change is happening, but a change in the overall frequency is.  This has really important implications for how we build our infrastructure. If a 100 year flood becomes a 10 year flood, we probably need better bridges.

So, we accept that extreme precipitation events happened in the past. But are they becoming more common?

The Learn More link [pdf] from the slideshow shows an increasing trend in heavy rainfall events in the US, especially since the 1980s.

Like most of the other indicators I’ve discussed in this series, an increase in heavy rainfall events doesn’t prove or disprove climate change by itself. But when combined with all the other indicators of climate change we have, it’s hard to argue that climate change isn’t happening.

If you want to read more about how climate change is expected to influence precipitation, I recommend CO2Now’s explanation.

This was my aunt’s last question from the EPA slideshow and IGBP information I sent her. I’ll add one more post to this series summarizing some of the common themes, my thoughts on climate skeptics like my aunt, and my aunt’s response to the series.

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