Frequently Asked Questions

We’ll be adding more questions as we go along. Visit our Climate Matters Blog to ask us for more questions.


Climate Basics

  • What is climate?
  • What factors determine climate?
  • What is the difference between weather and climate?
  • What are climate models?
  • What factors affect the earth’s radiation balance?
  • What is radiative forcing?
  • What are greenhouse gases?
  • What is the greenhouse effect?
  • Do all greenhouse gases have the same ability to warm the planet?
  • What are ecosystem services? Who depends on them?

Human-Induced Climate Change

  • Are humans affecting the greenhouse effect?
  • How do current greenhouse gas concentrations compare to previous levels?
  • What happens to anthropogenic emissions of carbon dioxide?
  • What is adaptation to climate change?
  • What is mitigation of climate change?
  • Are species at risk from climate change?
  • What are the consequences of melting permafrost?
  • Can nuclear energy help offset carbon emissions?
  • What is the Kyoto Protocol?

Oceans and Climate

  • Does the ocean store carbon dioxide?
  • Does the ocean take carbon dioxide out of the atmosphere or release carbon dioxide into it?
  • What determines the partial pressure of carbon dioxide in surface waters of the ocean?
  • What determines the exchange of carbon dioxide between atmosphere and ocean?
  • Is sea level rising?
  • What causes sea level rise?
  • Why do we care about rising sea levels?

Climate Basics

What is climate?

Climate is the average state of the weather, defined over a period of years, at a given location and time. This state is characterized by the mean values, and the typical range, of several weather variables, including wind, temperature, precipitation, humidity, cloudiness, pressure, visibility and air quality.

What factors determine climate?

Climate is determined by the solar energy that enters the earth system and how it is distributed. The radiation balance is the difference between the energy that enters the system and the energy that leaves it. This balance affects the temperature of the sea surface and air and causes the evaporation of water from oceans and land surfaces, thus contributing to cloud formation.

Because the earth is not a perfect sphere, more solar energy arrives to a given surface area in the tropics than at higher latitudes. As a result, energy is transferred from equatorial to higher latitudes via atmospheric and oceanic circulations, including wind, storm systems and ocean currents. This circulation powers the climate system.

What is the difference between weather and climate?

Weather is the state of the atmosphere at a given place over a short period of time (typically up to two weeks). This state is defined by characteristics such as temperature, precipitation, wind, humidity, pressure, visibility, and air quality. Climate, on the other hand, refers to the average state of weather at a given place over a long period of time (usually at least a decade). Therefore, climate is defined by averages, trends, and ranges, rather than by specific values. For example, while the exact temperature on a particular day is important in defining weather, it is not as important for climate; instead, a region’s climate is better defined by average temperature, and the range of the highest and lowest temperatures, over two or more decades.

It is also important to note that while weather is comprised primarily of atmospheric variables, even if they are affected by land and ocean, climate refers to a much larger system including oceans, ice, and land surface.

What are climate models?

Climate models are an important tool to study Earth’s climate. Because climate is such a large and complex system, computer models help scientists to synthesize and condense this system’s many processes, from ocean currents, winds, to precipitation. In addition, climate models allow scientists to experiment with the climate, which is, of course, impossible in real life: in a model, variables can be changed, results can be recorded, and eventually a picture of possible future climates can emerge.

Because of climate models allow us to project future climate, they play an important role as a guide for decision makers. See here for more information on climate models.

What factors affect the earth’s radiation balance?

While all the energy that enters the earth’s system comes from the sun, about 30 percent of the solar radiation that reaches the earth’s atmosphere is reflected back to space by light-colored features, including clouds, dust, deserts and ice. The ratio between solar radiation reflected back to space and incoming solar radiation is called albedo.

The energy that remains in the system is absorbed by the earth. To balance this, the planet radiates heat that is, on average, equal to the radiation it absorbs. Unlike solar radiation, which is primarily in the form of shortwave visible light, the longwave heat energy is trapped by greenhouse gases that occur naturally in the atmosphere.

There are three fundamental ways to change the radiation balance of the planet:

  • Changing the incoming solar radiation, by changes in the earth’s orbit or by changes in the sun itself.
  • Changing the fraction of solar radiation that is reflected back to space (albedo).
  • Changing the concentration of greenhouse gases that trap longwave radiation emitted from the earth.

Climate changes in earth history are thought to have occurred due to all three of these processes.

What is radiative forcing?

Radiative forcing is the extent to which the energy balance of the earth-atmosphere system is forced away from its normal state in response to changes in the factors that affect the climate. Factors such as albedo (the ratio between reflected and incoming solar radiation and that reflected) and the presence of greenhouse gases alter the balance between incoming solar radiation and outgoing heat energy.

What are greenhouse gases?

By preventing longwave radiation from leaving the earth’s atmosphere, greenhouse gases trap energy and keep our planet warm. The most common greenhouse gases are water vapor and carbon dioxide (CO2). Methane (CH4), nitrous oxide (N2O), ozone (O3), and halocarbons such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) also contribute to the greenhouse effect by trapping infrared energy. While many of these gases occur naturally in the atmosphere, their concentrations have increased dramatically since the Industrial Revolution as a result of burning fossil fuel.

What is the greenhouse effect?

To balance the incoming solar radiation, the earth emits energy that is, on average, equal to the amount of energy it absorbs. Because the earth is much colder than the sun, it radiates at much longer wavelengths, i.e., in the form of heat.

This heat energy is absorbed by naturally occurring gases in the earth’s atmosphere, including water vapor and carbon dioxide. Like the earth, these gases radiate in the infrared, so energy is transmitted in all directions, including back to earth. Known as the greenhouse effect, heat is trapped by the atmosphere. Consequently our planet is roughly 33o Celsius warmer than it would be in the absence of an atmosphere, thus rendering it habitable.

Do all greenhouse gases have the same ability to warm the planet?

Some greenhouse gases (GHGs) have greater potential to warm the planet than others. Because carbon dioxide is the most conspicuous greenhouse gas, the potential to warm is defined relative to that of CO2. A substance’s potential to warm the planet depends on its absorption of infrared radiation, the spectral location of its absorbing wavelengths and the atmospheric lifetime of the substance.

The Global Warming Potential (GWP) describes the radiative forcing associated with the release of one kilogram of a greenhouse gas relative to that of one kilogram of carbon dioxide. GWP estimates the relative impact of different greenhouse gases on the climate system.


  • Carbon dioxide has a GWP of exactly 1.
  • The GWP of methane is 25.
  • The GWP of nitrous oxide is 298.
  • The GWP of some halocarbons exceeds 20,000.

The GWP refers to a fixed mass of the greenhouse gas. Although the GWP of carbon dioxide is much less than that of other greenhouse gases, it is present in much larger quantities, and thus plays the major role in warming the planet.

What are ecosystem services? Who depends on them?

Ecosystem services are services that humans extract or obtain from ecosystems, such as food and water. Ecosystem services are often thought of as belonging to one of four categories:

  • Provisioning services, such as food, fresh water, fuel and wood.
  • Regulating services, such as climate regulation, flood regulation, disease regulation and water purification.
  • Cultural services, such as the recreational or religious value of ecosystems.
  • Supporting services, such as soil formation and photosynthesis.

Many ecosystems  and the services they provide are being degraded or used unsustainably. The impacts of ecosystem degradation are borne disproportionately by the poor. Carbon sequestration is a key regulating service of ecosystems like forests and very relevant for climate change.

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Human-Induced Climate Change

Are humans affecting the greenhouse effect?

Since the time of the Industrial Revolution, human activities have dramatically increased the concentration of greenhouse gases in the atmosphere. While this increase hasn’t changed the mechanisms of the greenhouse effect, the amount of energy retained by the earth system is greater, and the radiation balance has been altered. In this way, human activities have enhanced the greenhouse effect.

How do current greenhouse gas concentration compare to previous levels?

Atmospheric greenhouse gas concentrations have increased since the Industrial Revolution, primarily as the result of human activities.

  • Atmospheric carbon dioxide has ranged from 275 to 285 parts per million (ppm) in preindustrial times (AD 1000 – 1750), but has risen 36 percent to 379 parts per million in 2005. This increase is primarily due to the burning of fossil fuels associated with transportation, construction and manufacturing. Deforestation also releases CO2 and reduces its uptake by plants.
  • Similarly, atmospheric methane has varied between 400 and 700 parts per billion (ppb) over the last 650,000 years. In 2005, the global average abundance of CH4 was roughly 1,774 parts per billion. For the most part, atmospheric methane has increased as a result of human activities related to agriculture, natural gas distribution and landfills.
  • Since 1750, atmospheric nitrous oxide is estimated to have increased from 270 parts per billion to 319 parts per billion in 2005, primarily as a result of fertilizer use and the burning of fossil fuels.
  • The growth of tropospheric ozone varies regionally, yet the IPCC notes that concentrations have grown by 10 to 35 percent over the last 30 years in many places around the world. Tropospheric ozone is produced when sunlight reacts with air containing carbon monoxide, hydrocarbons, and nitrogen oxides, directly at the source of the pollution or many kilometers downwind. Tropospheric ozone is distinct from that found much higher in the atmosphere that forms the protective ozone layer.
  • Chlorofluorocarbons (CFCs) are now regulated by the Montreal Protocol on Substances that Deplete the Ozone, and their concentrations have decreased in the atmosphere since the protocol entered into force in 1989. However, the use of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) has increased dramatically, primarily because they frequently have replaced CFCs in refrigerants and solvents.
  • Water vapor is the most abundant and, indeed, the most important greenhouse gas in the atmosphere. Human activities have had a very small effect on the presence of water vapor in the atmosphere, though by changing the climate, humans have the potential to indirectly affect the amount of water in the atmosphere.

What happens to anthropogenic emissions of carbon dioxide?

About half of the carbon dioxide emitted into the atmosphere by human activities remains in the atmosphere, while the rest is taken up by land vegetation and by the ocean in roughly equal measures. Terrestrial plants consume carbon dioxide through photosynthesis. The magnitude of terrestrial uptake depends primarily on changes in land cover and land use, such as deforestation or increased urbanization.

What is adaptation to climate change?

Adaptation to climate change refers to adjustments in natural or human systems in response to actual or expected climate changes or their effects. The impacts of climate change may be harmful or beneficial to people, economic activities and/or the environment.

The extent of the impacts associated with climate change on ecosystems, regions and sectors of the economy depends not only on the sensitivity of those systems to climate change, but also on their ability to adapt. Adaptation to climate change may be done after the climate impacts have been observed (such as migrations following a rise in sea level) or proactively, anticipating potential impacts.

Some examples of adaptation to climate change include:

  • Improving water use efficiency, planning for alternative water sources (such as treated wastewater or desalinated seawater), and making changes to water allocation.
  • Improving early warning systems and flood hazard mapping for storms.
  • Improving weather advisories to alert the public.

Read here about examples of adaptation strategies in Indonesia and London. In addition, The IPCC Fourth Assessment Report (AR4) from Working Group 2, “Impacts, Adaptation and Vulnerability,” is an excellent resource.

What is mitigation of climate change?

Mitigation of climate change refers to social, technological and economical changes and substitutions that reduce atmospheric concentrations of greenhouse gases (GWGs). Atmospheric concentrations can be reduced by decreasing emissions directly or by increasing the “sinks” that absorb greenhouse gases.

Natural sinks of greenhouse gases include soil, the oceans and other bodies of water, and vegetation and other biological systems. Two strategies to increase carbon storage in terrestrial ecosystems are reforestation and avoiding further forestation.

Carbon capture and storage reduces emissions by capturing and storing carbon dioxide instead of releasing it into the atmosphere.

Capture from large point sources such as fossil fuel power plants is technically feasible though energetically costly. There is also ongoing research on air capture.

After capture, carbon dioxide can be stored in geological formations, in the deep ocean, beneath the seabed, or it can be transformed to inert mineral carbonates.

Greenhouse gas emissions can be reduced by:

  • Reducing energy use.
  • Increasing energy efficiency.
  • Switching to energy sources with lower greenhouse gas emissions.

Energy use can be reduced through behavior, by choosing to walk rather than drive for example. Some strategies to reduce use or increase efficiency include improving and using fuel-efficient vehicles (such as hybrids, semi-hybrids or electric cars) or implementing new energy technologies and energy efficiency measures.

Energy sources with lower emissions include nuclear power and renewable energies (such as hydrogen fuel cells, solar power, tidal and ocean energy, geothermal power and wind power).

For more information on the mitigation of climate change, see the IPCC Fourth Assessment Report (AR4) Working Group 3, “Mitigation of Climate Change.”

Are species at risk from climate change?

Species are at risk from climate change. Scientists estimate that, as a result of human activities, the current rate of extinction is up to 1,000 times greater than in the past. Climate change is already contributing to this increased rate of loss of biodiversity, and is expected to further augment it in the future. Climate change impacts biodiversity in many different ways. For example, increased temperatures can decrease the amount of habitat available to species, interfere with temperature-sensitive reproduction and migration events, and increase disease (e.g., by expanding the ranges of disease vector species such as mosquitoes).

What are the consequences of melting permafrost?

The extent of permafrost is predicted to decrease by 20 to 35 percent in the northern hemisphere by the mid-21st century, while the depth of permafrost is predicted to decrease by 30 to 50 percent by 2080. The melting of permafrost weakens the stability of the ground, which can significantly affect local infrastructure. In addition, there is concern that methane from decomposed animal and plant matter that is currently locked away under permafrost will be released as the permafrost thaws. Methane is a potent greenhouse gas, and its release could create a positive feedback loop in which increased temperatures lead to methane release, which in turn drives temperatures even higher.

Can nuclear energy help offset carbon emissions?

Yes, nuclear energy can help offset carbon emissions, as nuclear power plants do not emit greenhouse gases. However, other concerns about the use of nuclear power make it a controversial option to meet energy needs. These include the high initial cost of building nuclear power plants, the large amount of water needed for use as a coolant, and the difficulty of storing radioactive waste. For more information, see this summary from the U.S. Environmental Protection Agency.

What is the Kyoto Protocol?

The Kyoto Protocol, known formally as the Kyoto Protocol to the United Nations Framework Convention on Climate Change, is an international agreement that sets targets for the emissions of six major greenhouse gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).

Under the Kyoto Protocol, 37 developed countries plus the European community must reduce their emissions of these greenhouse gases by an average of five percent of 1990 levels between 2008 and 2012. Developed countries are largely supposed to meet these targets by reducing emissions within their own borders, but may also achieve credit through emissions trading with other developed countries and by contributing to emissions reduction projects in both developing and developed countries. To date, 182 parties have ratified the Kyoto Protocol, including all developed nations except for the United States. For a summary see The Kyoto Protocol – A Brief Summary (The European Commission).

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Oceans and Climate

Does the ocean store carbon dioxide?

The ocean is the largest global repository of carbon dioxide. It contains fifty times more CO2 than the atmosphere.

Does the ocean take carbon dioxide out of the atmosphere or release carbon dioxide into it?

Globally the ocean is a sink for atmospheric carbon dioxide because the global average partial pressure of CO2 in the surface ocean is less than that of the overlying atmosphere. Therefore carbon dioxide goes into the ocean. Mixing is very rapid in the atmosphere, so the amount of carbon dioxide, known as partial pressure, is fairly uniform throughout the globe. By contrast, the ocean mixes much more slowly and the partial pressure of carbon dioxide varies considerably in space and time. Regions that absorb atmospheric carbon are known as sinks, and regions that release it to the atmosphere are known as sources. Approximately 92 x 1015 grams of carbon enter the ocean each year and approximately 90 x 1015 grams of carbon go from the ocean to the atmosphere, leading to a net flux of carbon from the atmosphere to the ocean of approximately 2 x 1015 grams of carbon per year.

What determines the partial pressure of carbon dioxide in surface waters of the ocean?

Surface water concentrations depend on the temperature and the carbon balance of the water, which is itself affected by biological processes, such as photosynthesis and respiration. Water temperature affects solubility. As warm waters cool, they can absorb more carbon dioxide. As cool waters become warmer, CO2 is less soluble and is released to the atmosphere. Ocean water at depth is rich in carbon and regions in which deep waters rise to the surface due to mixing or upwelling tend to have high concentration of carbon dioxide. Photosynthesis can only occur in the illuminated upper ocean, and it consumes carbon dioxide. If the partial pressure of CO2 in the ocean exceeds the partial pressure of the overlying atmosphere, the flux goes from ocean to atmosphere.

What determines the exchange of carbon dioxide between the atmosphere and the ocean?

The exchange of CO2 between the ocean and the atmosphere depends on the difference in partial pressure between the ocean and the atmosphere and an exchange coefficient. The exchange coefficient depends on temperature and the turbulence in the water boundary layer. It is usually expressed as a function of wind speed. Although wind speed is an imperfect proxy for turbulence, it is easy to measure and, therefore, we have global maps of wind speed.

Is sea level rising?

According to the 2007 Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4), there is strong evidence that global sea level rose gradually in the 20th century and is currently rising at an increased rate. After little change up to 1900, the average rate of global average sea level rise for the 20th century was 1.7 ± 0.5 millimeters per year.

The IPCC predicts a conservative rise in the global average sea level between 0.18 and 0.59 meters (between 0.6 and 2 feet) in the next century. The range depends on the degree of warming, which in turn depends on choices made regarding energy choices, economic development and consequently greenhouse gas emissions.

This estimate is conservative, as the models do not include all the non-linear processes which could accelerate the rate at which ice sheets melt, such as the impact of warming ocean temperatures below and around ice shelves (that buttress land-based ice sheets) or the acceleration of ice sheet movement due to meltwater between the ice sheet and the underling rock. Other sources predict 2100 sea levels to exceed present values by 0.8 to 2 meters.

What causes sea level rise?

Sea level rise is caused by thermal expansion of the oceans (water increases in volume as it warms) and the melting of land-based ice. In the 20th century, sea level increases have been due primarily to heat expansion and the melting of mountain glaciers. The potential for significant sea level increase depends on the likelihood of the melting of the polar ice sheets, which contain 99 percent of land-based ice.

The rate of sea level rise varies regionally because wind or ocean currents can carry water to or away from land, independently of the volume of water in the ocean. Likewise land can sink locally (subsidence) or rise due to volcanic or tectonic activity. Land can sink because of groundwater extraction. Land can also rise in the slow rebound in response to having lost the weight of ice from the last Ice Age.

Why do we care about rising sea levels?

Present-day sea level change is of considerable interest because of its potential impacts on human populations living in coastal regions and on islands. In addition to the loss of low-lying land that would be submerged, rising sea levels would:

  • Submerge low-lying land.
  • Accelerate coastal erosion.
  • Increase the risk of floods.
  • Increase intrusion of salt-water into aquifers.

Even small increases in sea level will lead to more flooding as a result of storm surges.

Both human development and natural habitats are at risk when sea level rises. More than 100 million people would be affected by one meter of sea level rise, mainly in Asia and areas of Africa, as densely populated mega-deltas are especially vulnerable to sea-level rise.

More information may be found in the IPCC Fourth Assessment Report (AR4) Working Group 1 “The Physical Science Basis,” Chapter 5 – Observations: Oceanic Climate Change and Sea Level.

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