21-5 Factors Affecting the Earth’s Temperature

Can the Oceans Store More CO2 and Heat? We Do Not Know

The oceans help moderate the Earth’s average surface temperature by removing about 29% of the excess CO2 we pump into the atmosphere as part of the global carbon cycle. They also absorb heat from the atmosphere and slowly transfer some of it to the deep ocean, where it is removed from the climate system for long but unknown periods of time. Ocean currents on the surface and deep down are connected and act like a gigantic conveyor belt to store CO2 and heat in the deep sea and to transfer hot and cold water from the tropics to the poles.

Evidence suggests that large changes in the speed of the ocean currents in this conveyor belt, and its stopping and starting, contributed to wild swings in northern hemisphere temperatures during past ice ages. If this loop of currents should slow sharply or shut down, northern Europe and the northeast coast of North America would experience severe regional cooling. In other words, global warming can lead to significant global cooling in some parts of the world, with the climate of western Europe possibly resembling that of Siberia. Disruption or significant slowing of the loop would also disrupt other parts of the world with floods, droughts, severe storms, and searing heat.

How Might Stored CO2 Affect Water Chemistry? Ocean Acidification

In an acidic ocean, calcium carbonate would be less available to be precipitated into the shells and skeletons of marine organisms. This would directly affect a wide range of shell-forming organisms including molluscs, corals, and protists, as well as all the organisms that rely upon them. It could affect the larval development of marine organisms. It could affect the respiration of fish. Scientists are particularly concerned that corals would be negatively impacted since these animals have large requirements for calcium carbonate to build their exoskeletons, and since corals are already stressed by warming seas.

How Might Changes in Cloud Cover Affect the Troposphere’s Temperature? Another Uncertainty

One of the largest unknowns in global climate models is the effect of changes in the global distribution of clouds on the temperature of the troposphere. Warmer temperatures increase evaporation of surface water and create more clouds. These additional clouds can have a warming effect (positive feedback), by absorbing and releasing heat into the troposphere, or a cooling effect (negative feedback) by reflecting more sunlight back into space.

The net result of these two opposing effects depends on several factors. One is how much water vapour will enter the troposphere as the Earth’s surface warms. In 2004, measurements verified that water vapour is increasing in the troposphere as the Earth warms. However, they found that increases in water vapour in the upper troposphere were not as high as many global circulation climate models have assumed. The effects of clouds on atmospheric temperatures also depend on whether it is day or night. Other factors include the type (thin or thick), coverage (continuous or discontinuous), and altitude of the cloud, and the size and number of water droplets or ice crystals formed in clouds.’

How Might Outdoor Air Pollution Affect the Troposphere’s Temperature? A Temporary Effect

Aerosols (microscopic droplets and solid particles) of various air pollutants are released or formed in the troposphere by volcanic eruptions and human activities. They can warm or cool the atmosphere as well as hinder or enhance cloud formation, depending on factors such as their size and reflectivity.

Climate scientists do not expect aerosol and soot pollutants to counteract or enhance projected global warming very much in the next 50 years for two reasons. One is that aerosols and soot fall back to the Earth or are washed out of the lower atmosphere within weeks or months, whereas CO2 and other greenhouse gases remain in the troposphere for decades to several hundred years. The other is that aerosol inputs into the troposphere are being reduced—especially in developed countries.

Can Increased CO2 Levels Stimulate Photosynthesis and Remove More CO2 from the Air? A Temporary and Limited Effect

Some studies suggest that more CO2 in the troposphere could increase the rate of plant photosynthesis in areas where there are no other limiting factors such as lack of water or soil nutrients. This would remove more CO2 from the troposphere and help slow atmospheric warming.

However, recent studies indicate that this CO2 removal would be temporary for two reasons. One is that it would slow as the plants reach maturity and take up less CO2 from the troposphere. The other is that carbon stored by the plants as organic compounds would be returned to the troposphere as CO2 when the plants die and decompose or burn

How Might a Warmer Troposphere Affect Methane Emissions? Accelerated Warming

Global warming could be accelerated by an increased release of methane (a potent greenhouse gas) from two major sources. One is bogs and other wetlands and the other is ice-like compounds called methane hydrates trapped beneath the Arctic permafrost. Significant amounts of methane would be released into the troposphere if the permafrost in tundra and boreal forest soils partially or completely melts, as is occurring in parts of Canada, Alaska, China, and Mongolia.