Six major effects of greenhouse gases are as follows: 1. Deep Ocean Currents 2. Cloud Behaviour 3. Fertilisation Effect of Higher CO2 Levels 4. Biosphere Response 5. Poleward Migration of Species 6. Other Uncertainties!
1. Deep Ocean Currents:
The disruption of deep ocean currents by global warming could have profound consequences on the global climate. These currents, which act as a great conveyor belt carrying roughly 20 times the volume of water flowing through all the world’s rivers, depend on a delicate balance of salinity and temperature to drive them—a balance that may be shifted by warmer temperature in the polar regions and consequent changes in the amount of polar ice.
Evaporation from prevailing winds across the Atlantic brings rain to Europe and Asia, but raises the Atlantic’s salinity. Chilled by Arctic cold in the North Atlantic, this salty water increases in density and sinks to the bottom of the ocean, where it travels south in a huge circuit that takes it around Africa and through the Indian Ocean to the Pacific Ocean.
The Pacific’s less saline waters make their way in a shallower current back along a roughly similar route that skirts several continents, ultimately balancing the oceans’ salt budget and warming the European landmass in the process.
An infusion of less salty water, as might be brought on by the melting of Arctic ice, could cause the deep ocean currents to stall since surface waters could not gain the density needed to sink. Some suggest that this stalling of the ocean current systems has happened in the past, leading to dramatic effects on the climate.
According to this theory, the rapid changes in climatic conditions that have signaled the end of recent ice ages may have been precipitated by shifts in dip currents.
The role of the oceans in taking up or supplying atmospheric CO2 is another key uncertainty in the attempt to model greenhouse effects. Of the roughly 8.5 billion metric tonnes of CO2 emitted yearly by humans from fossil fuel burning and deforestation, only about half remains in the atmosphere.
The rest is taken up by the various links in the planet’s carbon cycle, including land-based ecosystems and the oceans. The terrestrial biosphere, including all plants—alive and dead—contains about 2.5 times as much C02 as the atmosphere. But, the world’s oceans contain about 50 times as much CO2 as the atmosphere and, each year, over 200 billion metric tonnes cycle in and out of the oceans through gas exchange at the sea surface.
2. Cloud Behaviour:
The behaviour of clouds in future greenhouse scenarios is also a major source of uncertainty in global climate models. Clouds play an important role in regulating the earth’s energy budget. They routinely cover about half the earth and account for a good deal of the 30 per cent of indirect sunlight that the earth reflects into spaces.
But clouds do not merely reflect solar radiation from the earth; they also absorb some of the heat radiated by the earth and reflect it back to the surface below. In other words, they also exert a greenhouse effect of their own.
Currently, averaged over the globe, the reflective property of clouds dominates their greenhouse effect and, thus, they exert a substantial, net cooling effect on the earth.
Both the reflective effect and the greenhouse effect of clouds are large compared with the greenhouse effect exercised by C02 and other trace gases in the atmosphere.
Recent data from a satellite-borne experiment show that, on a scale where the warming effect from C02 doubling is 1, the cloud warming effect is currently 7. Therefore, the result is a net cooling effect four times larger than the expected warming that would be caused by a doubling of C02.
3. Fertilisation Effect of Higher C02 Levels:
Anticipating how this balance will change is complicated by the many competing influences that could pertain under greenhouse conditions. For instance, photosynthesis rates are known to be sensitive to atmospheric C02 level.
The effect known as C02 fertilisation can cause an average 30 per cent increase in growth rates in many, but not all, species when C02 concentration is doubled. Recent data suggest that this effect is temperature-dependent and could be increased up to three-fold by a 3°C rise in air surface temperatures.
Further, there is evidence that C02 enrichment helps some plants minimise water loss, presumably making them more drought-tolerant—an important consideration if the drier conditions predicted by climate models for many mid-continent areas come to pass. If all these favourable effects of greenhouse conditions were realised, a net increase in plant matter might increase the carbon uptake of the biosphere and act as a negative climate feedback, helping to alleviate further C02 build-up.
Indeed, there is evidence that the annual carbon uptake in the Northern Hemisphere has increased during the growing season in recent years, perhaps indicating greater carbon storage.
This potential rise in methane production is particularly worrisome. Methane is 20-30 times more efficient than carbon dioxide at trapping heat, and its concentration in the atmosphere has been increasing more than 1 per cent per year.
Within 50 years, it may be the most significant greenhouse gas. Large methane releases from warmer tundra—or from other sources sensitive to warming, such as deep ocean deposits—could create a major climate feedback.
4. Biosphere Response:
Predicting how living systems will respond to global warming and what climate feedbacks they will exercise may be just as difficult as solving the riddles of ocean dynamics or cloud behaviour and provide still another major source of uncertainty in climate modeling. Terrestrial ecosystems play an important role in the global carbon cycle, storing about 100 billion metric tonnes of carbon each year in plant materials through the process of photosynthesis and releasing about the same amount through the processes of respiration and decay.
This annual flux is equivalent to almost 30 per cent of all the carbon held in the atmosphere, so even minor changes in the balance between photosynthesis, respiration and decay could affect the level of atmospheric C02 quickly.
5. Poleward Migration of Species:
Perhaps, the most severe disruption of the terrestrial carbon cycle could come as result of the rapidity of the expected global warming. One consequence of this warming will be a movement towards the poles of the range of many plant species. But will plants be able to migrate poleward as fast as their ranges shift? Species migration is usually a rather slow affair, especially for many long-lived tree species.
However, a rapid rise of 2°-5°C in global temperature in the next 100 years could well translate into a 500-1,000 kilometres range shift poleward a distance that many species will not be able to travel. This may result in the gradual extinction of many forest communities and, perhaps, a concomitant decrease in carbon storage rates.
6. Other Uncertainties:
In addition to the questions surrounding ocean, cloud and plant responses to global warming, there are other scientific uncertainties as well. Climate models do not describe sufficiently such important factors as the variation in solar output, the influence of volcanic and human-caused aerosols, the earth’s reflectivity, and the influence of soil chemistry on the carbon cycle, as well as several other minor factors.
In spite of these many deficiencies, today’s climate models provide a first estimation of global climate change with enough internal consistency and apparent relation to reality that they cannot be ignored, though their detailed predictions will require years of refinement.