This is part three of a series that documents a conversation on climate change between my aunt (who is unsure that climate change is occurring) and myself. So far we’ve talked more about trust than actual science, but that’s about to change!
My aunt had several questions after watching this EPA slideshow on climate change indicators. The tone of her questions (along with her admonishment that “figures lie and liars figure”) suggests that specific potential issues with individual data sets used to represent individual indicators erode the main conclusions of climate change science. I want to start by refuting that; the evidence for climate change comes from innumerable data sets supporting many, many indicators that together form an incredible number of lines of evidence in support of climate change.
But I think it’s still worthwhile to answer my aunt’s questions. Some of them reflect confusion over some difficult concepts. Here’s her first one:
The Drought slide said “from 2001 through 2009, between 30 and 60 percent of the United States experienced drought at any give time.” Then the Precipitation slide says “Average precipitation has increased in the United States and worldwide.” And, in fact, the graphic they show indicates by color coding that the majority of the areas (maybe even 80%) have increased precipitation. Now I realize that the Precipitation slide is for a period of 1901-2008 and the Drought slide is 2001 thorugh 2009, but what they are doing is using different period of time simply to make “their” point. What if both slides viewed the exact same years for the data?
Let’s take a look at those two slides in greater detail. The precipitation slide:
The drought slide has a generic image of a dry and dusty landscape and states
Over the period from 2001 through 2009, between 30 and 60 percent of the United States experienced drought conditions at any given time. However, the data for this indicator have not been collected for long enough to determine whether droughts are increasing or decreasing over time.
My aunt claims that “what they are doing is using different period of time simply to make “their” point.” Reading the slide captions clearly shows that this isn’t a deliberate case of obfuscation – we don’t have data over the same length of time or across the country for drought that we do for precipitation. We only have very good drought data for the US over the last decade, though we have learned a lot about drought in particular regions through climate proxies like tree rings.
You might be thinking that this doesn’t make sense. How can we have information on precipitation but not drought? Well, drought isn’t just a lack of rainfall – the timing, temperature, and soil type are all important to consider when determining drought.
I don’t blame you if you’re suspicious, though – there is something conceptually tricky going on here. At first glance these two slides seem to be presenting conflicting statements – how can we have more precipitation, but also more drought? What’s going on (pdf)?
As average temperatures rise because of climate change, the Earth’s water cycle is expected to speed up, increasing evaporation. Increased evaporation will make more water available in the air for precipitation, but contribute to drying over some land areas. As a result, storm-affected areas are likely to experience increased precipitation and increased risk of flooding, while areas located far from storm tracks are likely to experience less precipitation and increased risk of drought. Since the 1970s, drought-affected areas have increased on a global scale—more likely than not as a result of climate change caused by human activities.
So some areas get wetter while others get drier – a spatial change.
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.
So an area can actually be in a drought while average precipitation stays the same, or even increases, if the timing of the rain changes! This is easier to imagine with an example. Let’s say you live in Greensboro, North Carolina and this is your average monthly climate:
Imagine the precipitation doesn’t change at all, but the average temperature each month increases 2 degrees. Even though July is getting the same amount of rain, it’s drier because the water evaporates faster. Now imagine that the total yearly precipitation stays the same, but 3 inches of rain that might normally fall in July occurs in a June deluge. The yearly precipitation hasn’t changed, but July just got much drier.
Here’s another scenario we could see: Yearly precipitation increases by 10 inches, it gets 2 degrees hotter in April and May, and a 40% of the rain occurs in just 3 storms in August. So even though there’s more water, between the timing and the higher temperatures increasing evaporation, much of the year is actually drier than normal.
If someone is feeling adventurous, they could probably comb through old weather records and find years where this sort of thing has happened. Lest I open myself up for criticism here, just because we’ve had years like that in the past doesn’t mean climate change isn’t happening – we expect more drought and/or more rain and/or more extreme rainfall events (depending on where you live) with climate change – so much that it isn’t considered a weird year, it’s considered the new normal.
Even in a situation where precipitation increases aren’t from more extreme precipitation events, temperature could increase so much that it’s still drier. That’s why when I look at what global climate models predict for the southwestern United States, I consider both temperature and precipitation. An extra half inch of monsoon rain could very easily be offset by a temperature increase.
When we think about the future of water availability, we aren’t necessarily interested in how much rain falls, we want to know how much is there to be used. If we’re getting a lot of our rain in a few big storms, most of it is just going to run off (or do more damage as a flood than it’s worth). We also consider the effect of temperature: if it’s hotter, the water doesn’t stick around as long where we need it in soils and reservoirs and rivers – it returns to the atmosphere.
While we can’t directly answer my aunt’s question (What if both slides viewed the exact same years for the data?), the data is still in line with what we expect from climate change. More precipitation and more heavy precipitation events don’t necessarily mean fewer droughts.
- Drought, Floods, High Temps Consistent with Climate Change Projections (yubanet.com)
- Data suggests Iowa realy is getting wetter (gazetteonline.com)
- Jersey declares drought watch; Pa. next? (philly.com)
- Global Warming Makes Hurricanes Stronger, But Other Effects Remain Unclear (yubanet.com)