Greenhouse Effect

• How does the greenhouse effect work?

Visible light from the sun passes through the atmosphere and is absorbed by the Earth's surface - some of that energy is then emitted back to the atmosphere as heat. Greenhouse gases trap that heat, which would otherwise be released into space, raising the temperature of the atmosphere and, subsequently, the Earth's surface. Increases in greenhouse gases from human activities increase the amount of heat trapped by the atmposhere causing global warming and climate change.

• What are the most important greenhouse gases and their sources?

Water vapor - Water vapor contributes the most to the greenhouse effect and occurs in the atmosphere as a result of the natural cycle of water.

Carbon dioxide (CO2) - Carbon dioxide also cycles naturally between the atmosphere and living organisms. Plants and algae remove CO2 from the atmosphere via photosynthesis, while all living things release CO2 via respiration (i.e., breathing). Carbon dioxide also cycles back and forth between water on the Earth's surface (freshwater and the oceans) and the atmosphere. In addition to these natural processes, humans release large quantities of CO2 to the atmosphere by burning fossil fuels, deforestation, and other industrial processes.

Methane (CH4) - Methane is a natural byproduct of decomposition, but significant quantities are also produced via agriculture and animal husbandry as well as by fossil fuel production.

Nitrous oxide (N2O) - Nitrous oxide is released naturally from terrestrial soils and oceans, but substantial quantities are also generated from the use of nitrogen fertilizers in agriculture and through some industrial processes.

Other gases - A number of other natural and man-made gases also contribute to the greenhouse effect, including tropospheric ozone, and industrial gases such as halocarbons.

Aerosols - Aerosols are airborne particles within the atmosphere. Some aerosols, such as sulfate aerosols and black carbon aerosols are also produced by fossil fuel combustion. Sulfate aerosols tend to reflect incoming solar radiation, cooling the Earth's surface. Black carbon aerosols absorb, rather than reflect, solar radiation, which shades the Earth's surface, but warms the atmosphere.

• Is Climate Change a Natural or Human-Caused Phenomenon?

Our climate varies naturally over both short and long time-scales, but natural climate variability can be distinguished from human-caused climate change.

Scientists have conducted a number of attribution studies that compare observed changes in the global climate with those factors that are known to influence climate. These studies indicate that the climate change observed over the 20th century is due to a combination of changes in solar radiation, volcanic activity, land-use change, and increases in atmospheric greenhouse gases. Of these, greenhouse gases appear to be the dominant driver of climate change over the past few decades.

• How Do We Know that Atmosphere Increases in Greenhouse Gases are Due to Human Activity?

Some greenhouse gases, such as industrial halocarbons, are only made by humans, and thus their presence in the atmosphere can only be explained by human activity.
Naturally occurring gases such as CO2 and CH4 are generated by natural processes such as plant and animal respiration and decomposition. However, scientists can quantify the various sources (both natural and human) of such gases and measure their contribution to atmospheric concentrations. Current concentrations of the primary greenhouse gases (see above) cannot be accounted for without considering human activities, particularly the combustion of fossil fuels. Furthermore, global warming may increase the release of greenhouse gases from natural resources.

• Is There Any Connection Between Global Warming and the Ozone Layer?

Ozone is a greenhouse gas, and ozone depletion in the upper atmosphere is believed to have had a slight cooling effect on the global climate. Thus, ozone depletion in the upper atmosphere has not contributed significantly to global warming, although ozone depletion remains a concern because of its ability to block harmful ultraviolet radiation from reaching humans and wildlife.

Ozone in the lower atmosphere is an air pollutant and a health risk, and ozone in the lower atmosphere has increased over the past century contributing to global warming.

Therefore, though there is a relationship between ozone and climate change, ozone depletion is a concern largely due to its effects on UV-radiation rather than on climate.

• How Much Climate Change Has Been Observed to Date?   

Globally, surface air temperatures increased by approximately 1oF during the 20th century. Some regions of the world have experienced much greater warming; Alaska and the Antarctic peninsula, for example, have warmed by approximately 4oF over the same time period. Other regions of the world, such as the oceans of the Southern Hemisphere and the interior of Antarctica, have not experienced warming.

The observed warming over the 20th century was accompanied by a 10% increase in precipitation in the Northern Hemisphere and an increase in global sea-level of 4-8 inches.

• What Accounts for the Differences in Temperatures From Measurements at the Earth's surface and satellite measurements of the atmosphere?

Satellites are capable of measuring temperatures in the lower atmosphere (less than 6 miles in altitude), whereas surface temperature measurements are made just a few feet from the Earth's surface.

Surface temperature records show roughly twice as much warming since 1979 than satellite records (although surface temperature records date back to the mid 19th century, satellite measurements began only in 1979).

A report by the National Academy of Sciences in 2000 stated that both the surface temperature records and the satellite temperature records possessed systematic errors that may bias the data, which account for some of the differences between the two data records. Nevertheless, "the warming of surface temperature that has taken place during the past 20 years is undoubtedly real.

However, the National Academy of Sciences also concluded that there are real discrepancies between the surface and satellite records, which have yet to be sufficiently explained. It is often argued that urbanization has increased surface temperatures at the Earth's surface causing the surface to warm faster than the atmosphere, but surface and satellite trends for the world's most urbanized regions (e.g., North America and Europe) are nearly identical, indicating urbanization cannot account for the disparity. In addition, a recent analysis suggests that the difference between surface and satellite trends may not be as large as previously thought, but these results remain controversial.

• How Do Scientists Estimate the Climate of the Future and How Reliable are Their Projections?

Projections of future changes in climate are typically based on three sources of information:

- Knowledge of historical climate variability and change

- Scientific understanding of the climate system

- Computer models of the climate system that generate projections of future climate based upon a number of variables.

Of these three, climate models have received considerable attention. A number of different models exist and each represents the climate in a different way, resulting in large differences among models in projections of future climate change.

A number of current models do a reasonable job of simulating past climate variability (decades to centuries), but all such models perform poorly at modeling short-term climate variability (days-years) and regional climate variability.

The projections of climate models are also highly dependent upon the assumptions used regarding future trends in greenhouse gas emissions and atmospheric concentrations.

• What are the current estimates for 21st century climate change?

The latest Intergovernmental Panel on Climate Change projections for 21st century average global temperature increase is 2.5-10.4oF, based upon multiple climate models and multiple assumptions regarding future greenhouse gas emissions.

Regional warming may be greater or less than the global average. For example, temperature increases in the United States are projected to be approximately 30% higher than the global average. The Arctic is likely to experience the greatest warming.

Associated with this warming will be an increase in global average sea level of 4-35 inches, depending on the magnitude of warming.
Global precipitation patterns will also be altered by temperature increases. Generally, the hydrological cycle is expected to accelerate leading to increases in precipitation at the global level. However, these global increases may not necessarily balance the increased evaporation under warmer conditions, and some regions may experience a decrease in precipitation.

• What are the Projected Impacts of Climate Change?

Species in natural ecosystems will attempt to migrate with the changing climate, but will differ in their degree of success. Ecosystem productivity may decrease or increase, at least over the short-term.

Increases in temperature and changes in precipitation will have significant impacts on water resources, either reducing or increasing water availability along with increasing the risk of floods or droughts.

Coastal developments will experience additional sea-level rise that will interact with coastal storms to erode beaches, inundate land, and damage structures.

U.S. agriculture and forestry will likely experience mixed results with moderate warming, with increases in productivity likely in northern states and possible declines in southern states. However, at higher magnitudes of warming, the risk of more uniform adverse effects across the nation increases.

Human health may be affected by climate change through a number of mechanisms including extreme temperatures (i.e., heat waves), exacerbation of air pollution, severe weather, and increased spread of infectious diseases.

• Will There Be Any Benefits Associated with Climate Change?

Climate change may offer a number of benefits, depending primarily on the rate at which climate change occurs and the magnitude of temperature and precipitation changes in particular regions.

Current assessments indicate that agriculture and forestry in the United States are likely to benefit from low to moderate climate change, although these benefits will not be evenly distributed geographically, and some regions will experience damages.

With continued warming, however, benefits will likely peak and subsequently decline, and the effects of climate change for the nation as a whole in these sectors will turn negative.

Other benefits such as increased water availability, reduced energy demands, and greater ecosystem productivity may also occur in specific regions over the short or long-term. However, such benefits will likely be balanced by opposite effects in other regions.

• To What Extent Can Humans Adapt To Climate Change?

Some degree of adaptation will undoubtedly be necessary to respond to the coming climate change that is unavoidable.

Depending on the rate and magnitude of climate change, humans can invest in infrastructure and other societal systems to ameliorate its consequences.

However, different regions and sectors will differ in their ability to adapt. Natural ecosystems have inherent, but limited capability to adapt to climate change, which is further impeded by other human impacts to the environment such as development and habitat fragmentation. Even human societies, particularly developing countries, have limited resources to respond to the challenge of climate change.
Some climate related impacts are difficult to adapt to. For example, extreme weather events, such as storms and floods, are not easily ameliorated by adaptation measures.
Thus, investing in the reduction of greenhouse gases will offset necessary investments in adaptation in addition to protecting against those adverse effects of climate change for which adaptation is particularly difficult.

• How Much Do Greenhouse Gas Emissions Have to Be Reduced to Stop Climate Change?

Current atmospheric concentrations of greenhouse gases are projected to increase global temperatures by an additional 1oF in coming decades. Thus some degree of continued climate change is inevitable, despite efforts to reduce greenhouse gas emissions, but emissions reductions will aid in reducing the magnitude of that change and stopping human-induced increases in global temperatures.

In order to stop temperature increases, greenhouse gases in the atmosphere must be stabilized, meaning emissions of these gases must be reduced to such a level that they do not cause any additional increase in atmospheric concentrations.

The magnitude of emissions reductions necessary to achieve such stabilization depends on a number of factors including the level at which greenhouse gases should be stabilized and future patterns of fossil fuel use and emissions.

In its latest assessment report the Intergovernmental Panel on Climate Change estimated the magnitude of emissions reductions necessary to stabilize atmospheric concentrations of CO2 at a doubling of the preindustrial by the end of the century level for a broad range of scenarios for future greenhouse gas emissions. Given mid-range baseline projections for CO2 emissions, IPCC estimated that global CO2 emissions would have to be reduced by the end of the 21st century to 40-75% below baselines.

• What are carbon "sinks?"

"Sinks" are reservoirs that remove carbon dioxide (CO2) from the atmosphere and store it, sometimes by converting it to another compound. The two largest natural sinks for CO2 are the oceans and terrestrial vegetation, including forests. For example, forests remove CO2 from the atmosphere during photosynthesis, storing that carbon in their tissues. Keeping forests intact instead of cutting them down can prevent CO2 from being released and expanding forests can enhance removal of CO2 from the atmosphere. The Kyoto Protocol recognizes the preservation and enhancement of certain kinds of sinks and allows these to be counted as part of a country's efforts to meet its target.

• Is Planting More Trees a Way of Solving Global Warming?

Increasing the world's forest cover is uniformly considered to be a useful mechanism for mitigating atmospheric CO2 concentrations, because of the ability for plants to remove CO2 from the atmosphere through photosynthesis.

However, even a vigorous global reforestation program would not be sufficient to offset anthropogenic CO2 emissions from human sources.

Thus, reforestation may assist in reducing the rate at which atmospheric CO2 increases (and provide additional ecological benefits as well), but the stabilization of CO2 will still require direct reductions in CO2 emissions.

• What role do black carbon aerosols (also known as soot) play in global climate change?

Black carbon aerosols or soot are small, carbon-based particles that are emitted to that atmosphere as a by-product of incomplete/inefficient fossil fuel combustion (see glossary). Although not gases per se, these aerosols have similar warming effects on the global climate as traditional greenhouse gases such as carbon dioxide, methane, etc. Black carbon contributes directly to warming of the Earth’s atmosphere due to the ability of the particles to absorb incoming solar radiation, which is then reemitted to the atmosphere. Interestingly this also contributes to cooling at the Earth’s surface, because the absorption of solar radiation in the atmosphere contributes to a shading effect on the surface. However, the warming effect is estimated to be considerably larger than the cooling effect, and some estimates indicate that compared to the various greenhouse gases, the direct warming caused by black carbon is second only to carbon dioxide. Black carbon aerosols also influence the climate indirectly, by changing the reflectivity of ice and snow. Black carbon aerosols in the atmosphere eventually return to Earth, and when they occur on the surface of ice and snow, their dark color causes ice to absorb more solar energy than it otherwise would (i.e., black carbon reduces the Earth’s “albedo” - see glossary). This causes the Earth as a whole to reflect less solar energy in addition to promoting the melting of glaciers and other ice formations, particularly in the Arctic. Both of these effects contribute to global warming. Collectively, these direct and indirect effects make black carbon a major contributor to global climate change, yet carbon dioxide remains the dominant historical and future human influence on the global climate.

• Who is responsible for greenhouse gas emissions and climate change?

Once emitted, GHGs can remain in the atmosphere for many years, from approximately 10 years to thousands of years, depending on the gas. This means that emissions from a long time ago are still in the atmosphere and still affecting the Earth's climate system. Countries in the developed world have been emitting substantial quantities of GHGs since the start of the industrial revolution in the mid-18th Century. The United States, for example, is responsible for approximately 25 percent of the world's emissions of GHGs to date. However, although industrialization in other parts of the world has been delayed, emissions of GHGs from developing countries are rapidly catching up with those of the developed world, and some estimates indicate that emissions from developing countries, particularly those from China and India, will exceed those of the United States and Europe in coming decades. Determining responsibility for climate change necessitates consideration of these complex patterns of development, past, present, and future.