Climate Change 101

Frequently asked questions and answers are from the (CCRC), which addresses more questions on ecosystem effects and management options.

What is Climate?

Climate refers to the average meteorological conditions and patterns in a region over a long time period (classically 30 years according to the World Meteorological Organization). These meteorological conditions include measurements such as temperature, precipitation, and wind. In other words, climate can be described as the 'average weather'. Although weather can change rapidly from day to day and can be difficult to predict, climate is much more predictable. For example, the weather where we live dictates what we wear each day, which can change dramatically from one day to the next. However, the climate influences the type of clothes we have in our closet, which is generally consistent from year to year.

For more details, see the following resources:

(IPCC 2007, WG1, FAQ 1.1)
(IPCC 2007, WG1, FAQ 1.2)
( IPCC 2013, WG1, FAQ 11.1)

Read More:

What has caused the climate to change in the past?

Climate and climate variability are determined by three main factors (Chapin et al. 2002):

  1. The amount solar energy that reaches Earth. This is affected by changes in the Earth's orbit around the Sun and in the Sun's brightness.
  2. The chemical composition of the atmosphere. For example, changes in the amount of carbon dioxide and other greenhouse gases in the atmosphere affect how much energy (initially received as solar radiation) is trapped by the atmosphere. Energy that is not trapped is re-emitted into space.
  3. The reflectivity of Earth's surface and atmosphere. Surface features, such as ice sheets, increase the amount of solar radiation that is reflected back into space.

These factors and ways that they interact with one another have all contributed to the climate patterns and variability we see in earth's history.

Figure 1 - (US EPA)

However, the fact that climate has changed without human influence in the past does not mean that current climate change is following 'natural' patterns. The factor that best explains the observed global warming over the last century is the increase of CO2 and other greenhouse gases in the atmosphere, largely caused by the burning of fossil fuels and other human activity (IPCC 2013).

For more details, see the following resources:

(IPCC 2007, WG1, FAQ 1.1)
(IPCC 2007, WG1, FAQ 6.1)
(IPCC 2007, WG1, FAQ 6.2)
(IPCC 2013, WG1, FAQ 10.1)
(NASA Earth Observatory)

How do we know what climate was like in the past?

The factors that make up climate (such as temperature and precipitation) have been measured directly since the 1860's. However to understand what climate was like before this, scientists have to use 'proxies', or indirect measurements that have a well-defined relationship to temperature or other climatic variables. Some common proxies include:

  1. Ice cores - The ratio of oxygen isotopes in the ice can indicate the temperature at the time the ice was formed. Air bubbles trapped in ice can show what the greenhouse gas concentration of the atmosphere was at the time of formation.
  2. Ocean sediment cores - The remains of small hard-shelled creatures (foramnifera) are preserved in ocean sediments. The composition of their shells can reveal ocean temperatures at the time of their formation.
  3. Pollen records - Fossilized pollen grains can indicate what types of plants were present at the time the fossils were formed.
  4. Tree rings - The width of annual tree rings in temperate climates depend in part on soil moisture and temperature, and can therefore reveal information about these variables.

Past climate is often 'reconstructed' by looking at many of these measurements together.

More examples can be found at the following websites:

How do human activities affect climate?

Humans are affecting the climate by increasing the amount of greenhouse gases present in the atmosphere and by changing the surface characteristics of the land (see discussion about "albedo"). Activities such as fossil fuel burning, land-use change (e.g. deforestation), animal husbandry, and practicing fertilizer-dependent agriculture lead to increases in greenhouse gases including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). These contribute to the greenhouse effect and cause the surface temperature of the Earth to increase. Global atmospheric concentrations of CO2, CH4 and N2O have increased markedly since 1750 as a result of human activities, and now far exceed pre- industrial values (IPCC 2007, WG1, Ch. 2).

For more details, see the following resources:

(IPCC 2007, WG1, FAQ 2.1)
(IPCC 2007, WG1, FAQ 7.1)

See the video below from the to view changes in atmospheric carbon dioxide over history.





What is the greenhouse effect and what are the major greenhouse gases?

The greenhouse effect is the process by which certain gases in the atmosphere absorb and re-emit energy that would otherwise be lost into space (IPCC 2007, WG1, Ch. 1).

The Earth's transparent atmosphere lets sunlight through to warm the ground and the oceans (more when it is clear, less when it is cloudy). The Earth's warmed surface releases some of that heat in the form of infrared radiation, a form of light, but invisible to human eyes. Greenhouse gases in the atmosphere like carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor, absorb this infrared radiation and keep it from passing out into space. This energy is then reradiated in all directions, and the energy that is directed back toward the Earth's surface warms the planet.

The greenhouse effect is necessary for human survival, since without it the Earth's average temperature would be about 0 C (Swanston et al. 2011). However current concentrations of greenhouse gases are higher now than they have been for the past 800,000 years encompassing the history of human development, and are enhancing the greenhouse effect.

The 'greenhouse effect' gets its name from the fact that a fairly similar process takes place in a greenhouse. The glass panes on a greenhouse let incoming sunlight pass through them, but intercept some of the infrared light on its way back out, warming the interior (Climate Central 2009).



Figure 2 -  (Center for Climate and Energy Solutions)

For more details, see the following resources:

 (IPCC 2007, WG1, FAQ 1.3)

If climate varied naturally in the past, how do we know that humans are disrupting climate now?

The factor that best explains the warming trend over the last century is the increase of greenhouse gases in the atmosphere, which are caused by human activity. Scientists have examined many other potential causes of recent warming, such the Sun's variations in brightness, however, the magnitude of these effects is not sufficient to explain actual temperature observations on Earth. Human disruption is simply the only current explanation we have that fits the data (Lean 2009).

Climate models are an important tool for studying all of the various influences on the Earth's climate. These models are constructed to include many different measurements that could affect temperature, including solar output, volcanic emissions, and the effects of greenhouse gas concentrations in the atmosphere. When the effects of increasing levels of greenhouse gases are included in the models, as well as natural external factors, the models produce good simulations of the warming that has occurred over the past century. The models fail to reproduce the observed warming when run using only natural factors (IPCC 2007).

For more details, see the following resources:

(IPCC 2007, WG1, FAQ 2.1)
 (IPCC 2007, WG1, FAQ 9.1)
(IPCC 2011, WG1, FAQ 10.1)
 (NASA Earth Observatory)
 (NASA Earth Observatory)

Figure 3 -  (National Climate Assessment, 2014)




Is the rate of climate change greater now than the rate of natural climate change in the past?

Global average temperatures are currently increasing at greater rates than those that have occurred over the last million years of Earth's history. Global average temperatures started rapidly rising during the 1900's as the industrial revolution was accelerating and as humans started relying more heavily on burning fossil fuels for energy (see Figure 4).





Figure 4 -  (NASA, Goddard Institute for Space Studies)

For the past 100 years, we have clear records of the increasing rate of global temperature change. The decade beginning January 2000 and ending December 2009 was the warmest decade in the modern record (since 1880), with 2010 tied with 2005 as the warmest year on record (NASA 2011). From the 1910s to the 1940s, global average temperatures increased by 0.35 °C, and for an equivalent time period from the 1970s to 2006, global average temperatures increased by 0.55 °C (IPCC 2007), reflecting an increasing rate of change.

The Earth's temperature has undergone some large changes in the past; according to proxy data, global average temperatures did change by as much as 4°C to 7°C between ice ages and warm interglacial periods. However, this change took approximately 5,000 years (IPCC 2007). Current rapid rates of global climate change are clearly very unusual in the context of past changes. More abrupt shifts may have occurred in the past at smaller, regional scales due to ocean heat transport, but did not likely affect the global mean temperature (IPCC 2007).

For more details, see the following resources:

IPCC FAQs (2007):

NASA Earth Observatory:

What are albedo and feedback, and how does the Earth's surface affect climate?

Albedo is the reflectance of a surface, such as the earth's surface. Different earth land covers have very different albedo; for example, ice and snow are highly reflective (high albedo) while darker surfaces like vegetation do not reflect as much radiation (low albedo). Albedo plays an important role in global temperature change, since absorbed solar radiation warms the Earth's surface, whereas reflected radiation does not.

Feedback is a general term that encompasses all of the different forms of energy exchange between the land surface and the atmosphere. In the context of global warming, positive feedbacks enhance land surface warming; negative feedbacks enhance land surface cooling.

Albedo is one component of this energy feedback. Since different land covers have varied albedo, land use change can influence albedo and whether a land surface has a warming or cooling effect. As mentioned, snow has a very high albedo because it reflects more sunlight, and thus has a cooling effect on global temperatures (negative feedback). However, if snow melts and is replaced by darker vegetation, more sunlight is absorbed, which will add to a warming effect (positive feedback - IPCC 2007).

An emerging topic of interest related to albedo and feedback is the deposition of black carbon. Black carbon is a component of air pollution and is produced by combustion. It absorbs solar radiation and can greatly lower surface albedo if deposited on snow or ice. Black carbon is now thought to have played a major role in warming in the arctic, mainly by changing albedo and causing a positive warming feedback (Shindell & Faluvegi, 2009).

For a more details, see the following resources

NASA Earth Observatory:

Pew Center on Global Climate Change:

What are examples of climate changes that have occurred over the past century?

Global average surface temperatures have increased by about 0.74°C (1.33°F) between 1906 and 2005 (IPCC 2007), although the rate and magnitude of change differs with location and the period examined. While 0.74°C may not seem like a large temperature change, on a global scale this has huge implications for many of the earth's processes that effect ecosystems and humans. Even with this relatively modest temperature change, clear effects have been observed.

For example, in concurrence with observed temperature changes, in the 20th century there has been a nearly worldwide reduction in glacial mass and extent, a decrease in snow cover in many Northern Hemisphere regions, a decrease in Arctic sea ice thickness and extent, a decrease in the length of river and lake ice seasons, warmer ocean temperatures, and rising sea levels (IPCC 2007).

From 1900 to 2005, long-term changes in precipitation trends have also been observed, leading to wetter conditions in some areas (eastern North and South America, northern Europe, northern and central Asia) and drier in others (the Sahel, southern Africa, the Mediterranean, southern Asia). Widespread increases in heavy precipitation events have been observed, even in places where total precipitation amounts have decreased (IPCC 2007).

See the 'Ecosystem Effects' section of the FAQ's to read more about the implications of these changes.

For more details, see the following resources:

IPCC FAQs (2007):

US Global Change Research Program:

What do projections suggest for climate changes in the 21st century?

Average global surface temperatures are projected to increase by 1.1 to 6.4°C (2 to 11.5°F) by the end of the 21st century, relative to 1980-1990, with a best estimate of 1.8 to 4.0°C (3.2 to 7.2°F) (IPCC 2007). This large range in projections is due in part to the fact that we do not know whether greenhouse gas emissions will continue at their current levels, increase, or decrease. Temperature projections do vary regionally, with the highest projected warming taking place near the poles (USGCRP 2009). These temperature changes will continue to have effects on ice extent, snow cover, sea level, and many other factors. See the 'Ecosystem Effects' section of the FAQs for more detail.

Precipitation changes over the next century are more complex and uncertain, and differ within regions and subregions. Current models indicate that precipitation will generally increase in higher latitudes and in the tropical Pacific during the monsoon (rainy) seasons, and decrease in the subtropical dry belt. In the U.S. the Southwest is expected to become drier while northern regions may see an increase in precipitation (USGCRP 2009). Overall, the frequency and intensity of extreme weather events, such as heat waves, droughts, storms, heavy downpours, and heavy snowfall are projected to increase.

For more details, see the following resources:

IPCC FAQs (2007):

US Global Change Research Program: