There are reasons to believe that a climate stress is on its way at the present time. Lakes disappear, rivers run dry and deserts appear. An accentuation of this drying will again wipe out large tropical homeotherms such as hippopotamuses, elephants, rhinoceroses, etc. The origin of this climatic upheaval is to be found in the development of the railroads, around the years 1850. Railroads opened vast areas of virgin lands to agriculture and also allowed the easy transport of fossil fuels. During the latter half of the nineteenth century, there was a rapid development of agriculture in the lands of North America, Australia, New Zealand, South Africa and Eastern Europe. This "Pioneer Revolution" had hardly begun in 1850, yet large areas had been cleared and ploughed by 1900. The change was almost synchronous all over the world and led to the release in the atmosphere of very large quantities of CO2. This release took place before the fossil-fuel effect made a significant contribution. At the moment the pioneer revolution took place, the atmosphere of the earth contained 5.5 x 1011 tons of carbon and, by 1900, it contained 1.1 x 105 tons more. This represents one and a half times more CO2 released than has been produced by all the fossil fuels burnt up to 1950 (6 x 1010 tons carbon). As a result of the pioneer agricultural effect, the CO2 concentration of the earth’s atmosphere increased suddenly over a few decades in the late nineteenth century by about 10%. This effect was reinforced during the twentieth century by the growing use of fossil fuels, so that currently the CO2 levels in the atmosphere have been raised to 330 parts per million, that is, 22% above the pre-1850 level. It is generally agreed that the levels are increasing by about 1 part per million (PPM) each year. From the middle to the late 1800s, the temperature has increased about 0.5°C due to the pioneer revolution. By 2100, the global average temperature is expected to increase by as much as 3.5°C. Potential early stages of "greenhouse" conditions are beginning to be detected. It is estimated that a 1°C increase in surface temperature in regions of deep water would increase the atmospheric CO2 content by about 4.2 percent.
Modern oceans are stratified into a deep cold layer and a superficial warm layer but some mixing must occur otherwise the sea would have slowly filled to the brim with cold water. Oceanographers have for the last thirty years wondered about the forces at work in mingling warm and salty surface waters with colder and fresher supplies without however disturbing the stratification. Finding where and how the sea mixes is crucial to understanding how the ocean cools the planet by taking up heat and absorbing the greenhouse gas CO2. Sun and wind send cold, dense and fresh water into the deep sea. A mixing with the surface warm water must occur and cold currents must rise to mix with warm water but the amount of turbulent stirring in most parts of the ocean is ten times too weak to account for the mixing. In January 2001, an experiment was conducted to verify that the mixing is due to a phenomenon called salt fingering. When warm, salty water lies above cold, fresher water, centimeter-wide salt fingers are produced, mixing bodies of water hundreds of kilometers across, without stirring. Large bodies of water can mix without stirring, as interweaving fingers of water flow up and down between distinct layers of seawater. A subsequent stirring takes place because heat diffuses 100 times faster than salt in seawater. The salt-fingers are about a meter long and form a heat exchanger in which warm, salty water passes its heat to the colder, fresher water. By cooling down, the salty water becomes heavy and sinks. The fresh water warms up, lightens and rises. The experiment demonstrated that the mixing by salt-fingers was ten times more than the turbulent stirring measured at the same place could have caused.
The modern oceans contain about 60 times the amount of CO2 in the atmosphere and, if only 5 percent of the deep oceanic waters were to release this excess of CO2, the atmospheric content would increase by 25 percent, an amount larger than that estimated to result from combustion of the fossil fuels for the next 30 years. The combination of human-generated CO2 and that liberated from the oceans could trigger "greenhouse conditions" of major proportions and potentially within a relatively short time span. This is because the effect of warming in the Polar Regions is significantly greater than for the atmosphere of lower latitudes.
For years, atmospheric scientists have been wondering about where all the carbon is going. Each year, the US alone spews more than 5 billion tons of carbon dioxide emissions but about 20% of the total is absorbed by mainland US ecosystems. This sequestration offsets global warming but has remained unexplained until it became clear that the recently observed increased rainfall and humidity documented in the continental US might be the single most important factor favoring increased plant growth (14%), which in turn use the carbon to manufacture roots, stems, leaves and wood. Extra humidity allows plants to open wider the pores that allow carbon dioxide into their leaves, allowing photosynthesis to proceed more rapidly. Planting trees to counteract global warming will work well as long as rainfall is abundant and temperature is high.
The polar amplification of warming can affect the density stratification of the oceans. Modern oceans are stratified into a deep cold layer and a superficial warm layer and the cold Polar Regions cause this. This structure of the oceans will remain only as long as a continuous supply of cold dense water is provided from the poles. As soon as this supply is stopped, random mixing-followed by heating up-of the ocean waters will take place. By the first decade of this century, global temperatures may be warmer than any in the past 1000 years. Climatic warming will induce not only thermal damage to animal reproductive systems but also accelerated melting of the polar ice caps. This will result in a rise in sea levels. The rise is due to changes in the mass of the Antarctic and Greenland ice sheets, the melting of glaciers and above all the thermal expansion of the oceans, as the water warms. From 1993 to 1998, the rise was 3.2 mm per year. If warming increases, the rise may be monitored in cm per year. With a third of humankind living within sixty kilometers of a coastline, the number of refugees created will be unprecedented and it may be beyond our capacity to adjust easily to it.
There is a response to a warming world. In the Mediterranean regions, the leaves of most deciduous plant species unfold on average 16 days earlier and fall 13 days later than they did 50 years ago. In Canada, the poplars bloom 26 days earlier than 100 years ago. During the past 20 years, the growing season has become 18 days longer in Eurasia and 12 days longer in North America. Tree growth is now accelerated and flowers bloom on average 1 week earlier than they did 50 years ago. Our planet is greening, with the consequence that atmospheric CO2 is fixed and its atmospheric concentration diminishes. However, this greening affects animal life cycles. For example, insects become adults about 11 to 12 days earlier than 50 years ago. Frog calling occurs 10 days earlier than 100 year ago and birds show a 9-day shift toward earlier egg lying, from 1971 to 1995. The advanced leafing, flowering, fruiting and appearance of insects advances the availability of food supplies for sedentary birds to the detriment of the migratory birds. These now arrive too late in their breeding area and cannot compete anymore with the larger number of resident species as more of these survive the winter, breed earlier and have eaten all the available food.
Climate change also alters the geographic distribution of savannas, forests and tundra. Over the next hundred years, the steppe in Colorado and Texas will loose productive, palatable, drought-resistant buffalo grass (Bouteloua gracilis). This will be replaced by the native and exotic forbs and the ranchers will be unable to maintain the livestock production at its present level. North American forest will move 300 miles to the north. The boreal forest will largely replace the tundra: forbs in the tundra are nutritionally important to caribou during lactation, and lichens in the tundra are critical to the over-wintering caribou. Caribou herds will suffer reduction and Alaska’s native people will suffer.
It seems almost certain that the United States will warm during the coming decades. This warming will affect everything from the western snow packs that supply California with water to New England’s fall foliage. But this is about all that can be said. On a detailed level, the assessment often draws a blank. Whether the cornfields of Kansas will suffer drought or be blessed with abundant moisture is impossible to say. Overall productivity of American agriculture will remain high and is even projected to increase throughout the 21st century, but adaptation of agricultural practice to climate change will be the key.
The main challenges for avoiding excessive climate change are to curb carbon emissions from energy and transport systems, and to avoid deforestation. If we considerably exceed a critical threshold of CO2 concentration that might trigger an accelerated warming of the climate beyond our capacity to control, the duration of the next coming "greenhouse" would, in human terms, last an interminable period and its impact on life would be incalculable.