Looks like a duality of riches: CCNR#1
What elements of climate result in drought? And how is drought impacted by global warming? Here's a diary on surface water balance, and then some recent findings from climate change researchers on drought that the "primer" on water balance will help you better understand why reduction of greenhouse gas emissions is a social, economic, and political imperative.
A graphic telling the tale is below. The original study by NCAR (Meehl et al, 2007) was published in the Bulletin of the American Meteorological Society.
Surface Water Balance
The surface water balance depends on precipitation and evaporation as immediate impacts. Hydrologists and climate scientists look at the net of precipitation minus evaporation (P − E). If (P − E) is negative, there is a loss of water at the surface. If it's positive, there is a surplus of water at the surface.
But for agriculture and drinking, what happens to that surplus is important as well. Is the water stored in the soil, sitting on the surface as snow waiting to melt, or does it run off and get lost to local long-term storage as what we call runoff?
Below is a schematic from the University Corporation for Atmospheric Research's COMET Program of the water cycle, focusing on all the elements that go into runoff. I've highlighted three (really two) aspects of the water cycle that are affected by precipitation rates:
- Infiltration
- Percolation (once below the soil surface)
- Runoff (at the surface)
Why I've done that, follows the figure.
The importance of soil porosity and rain rates to subsurface soil moisture
Soil, by virtue of its limited porosity, can only allow in limited amounts of water per unit time, a.k.a. infiltration in the above diagram. Dry, sandy soil can initially allow over an inch of rain to infiltrate the soil surface per hour, while dry clay soil can only absorb about 0.25" per hour. Most dry soils have an initial range of 0.5" or so per hour. As the soils moisten, the rate that water infiltrates decreases. Rain rates from a typical low pressure system in the cold season usually runs upward of 0.1-0.2" per hour. All of that would get into dry soil. With warm season thunderstorms, rates can be 1-2" per hour or more, at least half of which would run off dry soil unless that soil were sandy.
Such precipitation will not recharge soil moisture subsurface long term storage aquifers, like the Ogalalla Aquifer in the Great Plains.
Impacts on soil moisture from global warming
One of the consequences of global warming is an acceleration of the atmospheric part of the hydrologic cycle, which will lead to heavier precipitation rates. This also potentially means less soil and aquifer moisture recharge for the same amount of precipitation than in the past. We've already seen evidence of this in many studies of precipitation data over the available record, in some places over the last 100+ years. Additional negative impacts on moisture storage available to crops, that can result in what is called agricultural drought include:
- Snow mass storage moving poleward and at higher elevations from where it was before.
- More rapid snow melt in spring where snow accumulates, resulting in more runoff and less soil moisture storage.
- Earlier green-up in the spring resulting in increased demand for water from the soil.
- Warmer temperatures resulting in higher evaporation rates, also reducing soil moisture.
The only potential benefit to soil moisture might result from more liquid precipitation where snowfall occurred before. This would increase soil moisture supplies in winter, when unneeded, and only replaces snow melt that would occur in the spring anyway.
The future of drought according to climate models
All of these effects and others I've not discussed here are accounted for as accurately as possible in global climate models, using conservative (in the "tending not to change" sense) methods based on observations of the current and recent past climate, characteristics of vegetation, soil, snow pack, topography, oceans (including having the ocean respond to the atmosphere, and thus to the increasing greenhouse gas levels), and so on.
Side note: An important impact of CO2 induced global warming (and indirectly, the impact of increases in water vapor in the atmosphere from that warming) is the melting of permafrost and the warming of the oceans. While the physical processes of melting and warming are included in climate models, the resulting release of methane (CH4) from those processes is not. This may lead to a significant underestimate of warming in climate models.
That being said, the graphic below shows the 10-year (decadal) average of a modified version of a drought index used by our National Weather Service, that describe U.S. drought severity, the Palmer Drought Severity Index (PDSI). The modified PDSI is calibrated by location to be used globally, using the 1950 to 1979 period. The data come from a 22-model ensemble of long climate simulations that are run for awhile to get them internally balanced, using the CO2 concentration from 1950. They are then run with observed and projected increases in CO2, using the A1B CO2 increase ("moderate") scenario, out to the year 2100.Greens and blues are wetter than normal, yellows and oranges are drier than normal. A PDSI of −4 or less indicates severe drought.
The graphic starts with 1950-59 in the upper left, then shows 1975-84 in the upper right. In those two ten year periods, the range in the PDSI is from about −3 to +3 generally. A few features stand out between the two:
- Drying of Australia
- Drying of the Middle East, Iran, and Afghanistan
- Moistening of western equatorial Africa and drying east of there in the Sudan, Ethiopia, and adjoining areas
- Moistening of the U.S. and Russian "breadbaskets" in the Great Plains and Ukraine, respectively. The 50s were dominated by a very dry period from about 1953-57, that rivaled the 1930s
The next two graphics from left to right are from the climate models for the decade we just finished 2000-09, and 2030-39 respectively. Drought increases significantly in the models over mid-latitudes in the northern hemisphere continents over time, and increases to well in excess of −4 values in many areas by 2030-39. For 2060-69 and 2090-99 (the bottom two images), huge areas of historic drought in excess of −8 PDSI appear and expand. A caveat: the report warns that because the basis for the PDSI is the 1950-79 period at each location, such large negative values pretty much lose their meaning, though they do represent unprecedented drought when and where they occur.
Effect on agricultural production
The effect of this drought on food production, if it were to occur, would be catastrophic. Below are the primary production regions during the 2000 or 2005 agricultural year for five primary crops: maize (corn), wheat, rice, soybeans, and potatoes.
Maize (corn)
Wheat
Rice
Soybeans (2005)
Potatoes (2005)
Note that almost all the crop areas lie in areas forecast to see drought not seen in over 100 years (and probably not in millenia, if this were to come true, but again with the usual caveats). But by 2030, the world population will rise from an estimated 6.88 billion people today (U.S. Census estimate) to anywhere from 7.5 to 9 billion people, according to a UN report done in 2004. Demand for food, energy, and water will increase by 50%, if nothing changes.
We'd be particularly hard-hit in our home country, the U.S. Wheat and corn production is centered in the area that may be hardest hit (soybeans as well, though that's not shown by area coverage). Europe, Russia, much of Africa, and southeast Asia (with rice production) would also be in dire straits assuming this scenario were even remotely true. Northern latitudes become considerably wetter, but that doesn't help us, or the Europeans (other than the Scandinavians), or the tropical and subtropical locations such as the South Americans in the Amazon, the South Africans/Zimbabweans/Botswanans/Angolans and so on.
Solutions?
Drought resistant strains of many crops are being developed. Perhaps the "veganification" of the western diet, and the prevention of the westernization of the developing country diet, might mitigate some of the inevitable food shortages if the drought scenario shown here comes to pass.
The ultimate solution, however, would be to prevent the warming from occurring, at least to this extent, in the first place. And given the current political climate, how likely do you believe that will happen?
If the political will isn't found to work on the economic and social problems that have created the global warming challenge, it's possible we'll be faced with this:
DISCLAIMER: This diary has been affected by my mood. I've been sick for the past 5 days. And the holidays are not my favorite time of year generally ... they remind me of where the global warming issue has its root cause.
Since I've been ill this past week, I'm going to cut myself a break ... and ask for your indulgence. What I will do is put up some links of climate blogs and their lead stories as of right now.
CLIMATE BLOG of the WEEK: Climate Progress
Climate Progress has a lede stating "Met Office finds "evidence for man-made warming has grown even stronger in the last year.". The Met Office is in the U.K.
Drought in the Amazon is covered in Climate Progress as well. There's an article from the Global Post that is very descriptive, and very disturbing.
There's an entry titled Farmer in the Times: "Climate change, I believe, may eventually pose an existential threat to my way of life." This is about a Minnesota farmer who has been having extraordinary trouble coping with the recent years' spate of climate extremes, particularly precipitation. This may be a preamble to a future SNAFU (you all know that acronym, yes?).
Climate Progress will be covering the proceedings in Cancun over the next two weeks, including some live blog coverage.
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Somehow, this quote seemed appropriate for tonight:
Violence is like a weed - it does not die even in the greatest drought. -- Simon Wiesenthal
And with that, I'll sign off.