A global temperature increases of 4° F (2° C) may not sound like much, but the increase will not be uniform. Because water vapor absorbs and blocks incoming solar radiation, areas with dry air will see more extreme temperature fluctuations. The interior of the continents and the polar regions would see more than double the average increase. In fact, based on a doubling of CO2 levels, the predicted mean summertime temperatures for the central US and central Russia range from 7° to 11° F above the current levels. Polar temperatures could increase by as much as 14° F. Midwinter temperatures in Antarctica in 1998 were already 9° above normal and arctic temperatures in the winter have risen 11° over the past 30 years.
In most of the same areas, rainfall will decrease significantly. The most often used model, operated by the Geophysical Fluid Dynamics Lab, does not output rainfall, but soil moisture which is more important in predicting crop production. In the great planes states the soil moisture could decrease by 50% which would lead to a major disruption in grain production. In order to keep up production, more irrigation would be required, however, much of the area is over the Ogallala Aquifer which is already being depleted at 12 billion cubic meters per year. The alternative, to switch to more drought tolerant crops such as soybeans, would lower yields and would not meet the world demand for wheat and corn. The onset of dryer conditions would not be gradual, but would be characterized by an increased frequency and severity of droughts with a tendency to have several good years followed by a year or more of extremely hot dry conditions. Rather than an orderly process of adapting to new conditions, farming without irrigation would become increasingly a game of roulette. Is there enough water to take care of the increased irrigation demands for the existing area plus the new areas that would need irrigation, plus the increased domestic demand, plus the increased demand for hydroelectricity to run air conditioners? The answer is no. Something would have to go, and it would most likely be agricultural needs since industry can pay a higher price for water than agriculture.
Could the decrease in crop yields be supplemented by an increase in yields from Canada? To some degree yes, but the areas in Canada that would become available for farming, do not have the rich soils of the great planes so the new land would not have an equivalent yield. A similar situation exists in Russia and China. The bread baskets of the world are for the most part located in the central part of continents, in the temperate latitudes.
As the map below shows, other areas that would experience a 50% reduction in moisture are Southwestern Europe, Northwest Africa, and Southwest Asia north of 30° N. These are all heavily populated areas, in some cases barely able to feed themselves under present conditions.
- Reduction in Global Soil Moisture Levels
- with 2X and 4X increase in CO2
Recent news: (from Grist Magazine July 31, 2000)
- China is enduring one of the worst droughts in its history, and many
experts are worrying that the nation is running out of water for its
1.3 billion people. Some 400 of China's 668 cities have declared
water shortages, which means that taps may work only a few hours a
day, if at all. At least 20 million Chinese citizens don't have
access to any running water, and another 200 million experience
serious water rationing or shortages. To make matters worse, much of
the freshwater that's available is polluted. Northern China is also
beset by the problem of desertification, with more than 900 square
miles of land turning into desert each year, thanks to excessive
grazing and logging. Overpopulation and climate change are also
worsening the nation's problems.
Source: Toronto Globe and Mail, Miro Cernetig, 07.31.00
There is historical evidence that such patterns are reasonable. After the last ice age there was a warm period, the Post Glacial Optimum, 4,000 to 8,000 years ago. At that time it appears that the North American grain belt was a dry grassland. In more recent times, the dust bowl of the of the 1930's gives us a glimpse of the future. The average temperature during July in 1934 was 4° F above normal, less than expected in the next century, but there was only half of the normal rainfall. Since that time, farming techniques have improved, soil and water conservation methods are widely known, but even these cannot cope with the extreme conditions of heat and drought expected.
Turning to the West Coast, the temperatures increases should be more moderate, and there would be a slight increase in rainfall. A temperature increase of a few degrees, however, will have a much larger impact on the snow pack. The Pacific Northwest and California depend on water stored as snow for their water supply for domestic use and irrigation. The dams and reservoirs are designed for the gradual melting of snow, and would not provide adequate storage to supply water all summer if the precipitation came as rain during the winter months with less snow and an earlier melt.
The Pacific Coast states are subject to a weather pattern known as the Pacific Decadal Oscillation which provides a revealing analog for the leverage that temperature has on snowpack and other factors such as forest fires. With a temperature variation between the warm and cool cycle of only 0.61° F, the snow depth changes by 32% and the area burned by forest fires increases by 113%.
The reaction of ocean currents to global warming is still an unsolved riddle with significant implications. The flow of ocean currents control much of the worlds weather by distributing heat from the tropics to the northern latitudes. There is evidence that a warming planet could disrupt this process by the influx of fresh water into the oceans from runoff and melting ice. There is evidence both from historical investigations, and from computer models that the flow pattern could break down quite suddenly if a critical threshold is reached. The result would be a rapid shift in climate and rainfall for areas dependent on these currents. Without the warming influence of the Gulf Stream, for instance, the climate of Northern England would resemble the climate at a comparable latitude on the west side of the Atlantic, roughly an area in Labrador, 700 miles north of Montreal Canada. As the graph shows, with a doubling of CO2 the predicted flow would be cut in half. Predicting the the resultant alteration in patterns of currents is the focus of current research.