what We Know: Underlying Processes

Living things don’t just respond to the climate—they affect it as well. Plants consume carbon dioxide and produce oxygen through photosynthesis. Earthbound plants take carbon dioxide directly from the air; drifting photosynthetic microorganisms called phytoplankton use carbon dioxide dissolved in water.

It is estimated that photosynthesis is a “sink” for around 60 billion tons of carbon every year, by far the strongest mechanism for carbon dioxide removal from the atmosphere. (This removal is almost exactly balanced by the respiration of animals, which combines oxygen with hydrocarbons to produce carbon dioxide and water vapor.) Increases in the level of carbon dioxide in the atmosphere could promote plant growth. If the planet’s vegetation grows stronger and more widespread, it could take in more of the atmospheric carbon dioxide, preventing a runaway greenhouse effect. This controversial “greening hypothesis” has led to more research exploring the connections between global climate and the earth’s biological systems. (See “Greening in the North” on this site to learn more about how climate change will affect vegetation.) This satellite image shows phytoplankton in the ocean waters off the southeastern United States. Phytoplankton, like terrestrial plants, consume carbon dioxide in the process of photosynthesis.

The biosphere is also the source of aerosols, such as spores, pollen, bacteria, and other particles. These aerosols scatter incoming radiation, affecting the energy budget. And some marine organisms produce sulfate particles, which act as condensation centers for cloud formation. As the number of such condensation centers increases, more, and consequently smaller, cloud droplets are formed. A cloud made of many small droplets is highly reflective and prevents solar radiation from reaching the earth. Any anthropogenic (human) production of sulphur could produce a similar effect, moderating a warming trend.

Evidences and Uncertainties

Many species live in very sensitive ecological niches, so even small changes in temperature or precipitation could drastically alter their ability to survive. Oak trees in the midwestern U.S., for example, may not tolerate an average temperature only a few degrees higher than current temperatures. And even increases in ocean temperatures of as little as 1°C over two or three days can cause coral—organisms particularly sensitive to long-term variations in climate—to lose their symbiotic algae, which are essential for their nutrition. When the algae die, corals are “bleached” and appear white. (See “Current Coral Bleaching Hot Spots” on this site to learn more.) Because all species are linked in complex webs of predator, prey, and habitat, impacts on one species always affect others—and it’s extremely difficult to predict how those effects will manifest themselves. Changing the life cycles of key species in food chains may well affect an entire ecosystem.

Additionally, the ability of many species to adapt to changing climate through migration is much different than it was in earlier centuries. Habitats and migration routes are now broken up by housing, industry, roadways, and other development. Species also need time to make adaptations, but the rate of climate change appears to be increasing: Over the last 16,000 years, the rate of increase in global temperatures has been about 1°C for every 4,000 years—and yet, some predictions now suggest that we may see another 1° increase over the next one hundred years. Some species may actually be helped by warmer temperatures, of course—but this may not necessarily be good news. Increasing the populations of some species may have serious effects on human health. Each of these pinwheel patterns centers on a small community of soybean farmers in eastern Bolivia. Roads and fields have subdivided the tropical dry forest.

For example, even small increases in global temperatures, especially if they’re accompanied by flooding, may drive an increase in the mosquito populations in tropical areas, leading to much greater transmission rates of diseases like malaria. (See “Risk of Malaria Transmission” on this site to learn about another potential health risk.) And again, changing the balance of species affects the way entire ecosystems function, with unknown consequences.