The Center for Climate and Energy Solutions is a source of reliable information about the causes and potential consequences of climate change. Here we provide an overview of fundamental facts and data and answers to frequently asked questions.
The world is undoubtedly warming. The Earth’s average surface temperature has increased by about 1.4°F (0.8°C) since the late 1800s. Since the 1970s, each decade has been warmer than the previous decade.
The 10 warmest years on record (since 1880) have all occurred since 1998, and all but one have happened since 2000. See a list of global average annual temperatures here .
The warming of the Earth is largely the result of emissions of carbon dioxide and other greenhouse gases from human activities. These activities include burning fossil fuels and changes in land use, such as agriculture and deforestation.
As a result, greenhouse gases are accumulating in our atmosphere. Carbon dioxide concentrations in the atmosphere since pre-industrial times have increased from 280 parts per million to nearly 400 parts per million.
The reason for the accumulation is simple: Human activities are putting more carbon dioxide than the planet’s vegetation and ocean can remove. A useful analogy is that of a bathtub, where the flow of water out of the faucet exceeds the flow through the drain, as illustrated in the U.S. Environmental Protection Agency’s (EPA) background materials  on the causes of climate change.
The warming we’ve observed has been driven, in large part, by the accumulation of greenhouse gases in the atmosphere.
The Fifth Assessment Report (AR5) from the IPCC summarizes: It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century… It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.
IPCC AR5, Working Group 1, Summary for Policy Makers, p.17 
Other factors capable of changing the climate, like volcanic eruptions and changes in the sun’s intensity, cannot by themselves explain the changes we’ve observed in the Earth’s climate. The figure below shows the outcomes of different computer simulations of climate (see caption for details).Only the simulations that included human influences exhibited warming similar to the observed temperatures around the globe during the last century.
The amount of warming that occurs by the end of this century depends on our choices now. If we don’t make much progress in curbing emissions, temperatures for the planet could rise between 4.7°F to 8.6°F (2.6°C to 4.8°C) by the end of the century, compared to the average temperature around the end of the 20th century (1986-2005). Warming in the United States is expected to be higher than the global average. Warming averaged across the country could be between 5°F to 10°F, assuming that emissions rates continue.
Although we have the opportunity to avoid some of this warming, we are still likely to face a number of impacts arising from climate change in the coming decades. In fact, we are already observing some of these impacts now.
These impacts pose challenges to infrastructure, businesses, and communities, particularly in countries already struggling to meet the basic food, water, shelter, and security needs of their citizens.
In addition, rapid warming can increase the risk of climate “surprises” or “tipping points.” Examples of these tipping points include the injection of methane into the atmosphere from thawing permafrost that could further accelerate warming, or the loss important ecosystems, such as large areas of the boreal or Amazon forests, that occurs as temperatures warm and precipitation patterns change. Although we don’t know when some of these tipping points might be crossed, continued warming would raise the chances that they could occur.,
We now have two jobs ahead of us.
The first is mitigation – using policy, technology and other actions to reduce the greenhouse gas emissions responsible for climate change. We need to transition to a lower-carbon economy . Steps along this path will include improving energy efficiency ; increasing the use of low- and zero-carbon energy sources such as wind , solar , and nuclear power ; and developing carbon capture and storage technologies .
The second is bolstering our resilience  to climate impacts– making sure that businesses and communities can withstand the changes in the climate that we can’t avoid.
"Global warming" refers to the increase of the Earth's average surface temperature due to a build-up of greenhouse gases in the atmosphere. "Climate change" is a broader term that refers to weather trends observed over relatively long periods of time (many decades or longer). Climate change can include many variables (temperature, precipitation, wind direction, wind speed) and different geographic scales (over a continent, within an ocean, for the Northern Hemisphere, for the planet).
Human activities that release carbon dioxide and other greenhouse gases into the atmosphere are largely responsible for recent climate change. The pattern of warming that we have observed, in which warming has occurred in the lower portions of the atmosphere (the troposphere) and cooling has occurred at higher levels (the stratosphere), is consistent with how greenhouse gases work – and inconsistent with other factors that can affect the global temperature over many decades, like changes in the sun’s energy. Although natural forces affect the climate (like volcanic eruptions and variations in the sun’s energy), they alone cannot account for the warming that has occurred.
Several pieces of evidence make it clear that greenhouse gas concentrations in the atmosphere are increasing because of human activities:
This evidence leaves no doubt that greenhouse gas concentrations are increasing because of human activities.
This illustration shows components of the climate system that would be expected to change in a warming world, and the changes they show that are consistent with warming (arrow direction denotes the sign of the change).
In addition to direct measurements of air temperature, we have a number of other changes (see figure) that are consistent with a warming planet. The evidence for these changes has grown stronger over the years.
The National Climate Assessment summarizes the state of our knowledge:
Evidence for climate change abounds, from the top of the atmosphere to the depths of the oceans. Scientists and engineers from around the world have meticulously collected this evidence, using satellites and networks of weather balloons, thermometers, buoys, and other observing systems. Evidence of climate change is also visible in the observed and measured changes in location and behavior of species and functioning of ecosystems. Taken together, this evidence tells an unambiguous story: the planet is warming, and over the last half century, this warming has been driven primarily by human activity.
National Climate Assessment, p.7
Climate change involves changes in long-term statistics of weather, but it does not mean an end to cold weather or to winter. Instead, it means that, averaged over many decades, cold winters and mild summers will become less frequent, and mild winters and hot summers will be more frequent. In fact, both of these trends have been observed over the past 50 years in the United States and globally. It is also important to remember that a cold winter for one location doesn’t mean a cold winter everywhere. The U.S. Midwest and East Coast experienced a relatively cold winter in 2013-2014, but California had one of its warmest winters ever recorded.
Projections for average global temperature increase this century range from about 2°F to around 11°F compared to temperatures in the late 1900s. However, at the higher latitudes, many locations are likely to warm by more than the global average (see figure).
The large range among projections stems mostly from different pathways in future energy use and greenhouse gas emissions. To keep warming to the lower end of the range, significant cuts in emission would need to be implemented immediately. In recent decades, the planet’s greenhouse gas emissions trajectory has been much more similar to the high end of the warming projections.
During the last 15 years or so, global mean surface temperatures have not warmed as rapidly as they had during previous decades. The Intergovernmental Panel on Climate Change (IPCC ) Fifth Assessment Report  (AR5) concludes that this is probably due to a combination of factors, including a redistribution of heat in the ocean, volcanic eruptions, and the recent minimum in the 11-year solar cycle.
“…trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”
IPCC AR5, Working Group 1, Summary for Policy Makers, p.5
Looking at the past temperature record or computer simulations of future climate shows that periods of less warming (or even slight cooling) can occur within longer periods of warming. In other words, the recent apparent “slowdown” in warming does not change the “big picture”  of our understanding of climate change, or our expectation for future warming.
There are some benefits that come with warming and increased carbon dioxide:
However, most studies show that damages caused by climate change far outweigh these benefits. Work supporting the Risky Business report  shows that in almost all regions of the United States, warming will create more problems than benefits.
According to the National Climate Assessment  and the IPCC Working Group 2 Report: Impacts, Adaptation, and Vulnerability , potential harm to individuals, communities, and businesses, include threats to:
The greenhouse effect is a naturally occurring process in the Earth's atmosphere that warms the planet. In the absence of a greenhouse effect, the average temperature at the Earth's surface would be approximately 60°F colder.
Visible light from the sun passes through the atmosphere and is absorbed by the Earth's surface, heating it up. That energy is then emitted back to the atmosphere as heat. Greenhouse gases in the atmosphere can absorb this energy, preventing it from escaping into space. This raises the temperature of the atmosphere and ultimately, the Earth’s surface. You can think of greenhouse gases as a blanket - and human-induced increases in greenhouse gas concentrations make this blanket thicker, warming the planet.
Source: National Park Service 
We have a complete discussion of the main greenhouse gases and their sources here .
The EPA also provides a wealth of information about greenhouse gas emissions and sources .
Yes. Polls of climate scientists show there is not an active “debate” within the field. For example, this poll  shows that 97 percent of scientists who specifically study climate systems agree that recent warming is real and is almost certainly caused by human activities.
There are plenty of important research questions debated by scientists. How fast will the ice sheets melt? How are changes in the jet stream related to climate change? But scientists agree on the fact that the planet is warming, and that human activities are an important driver.
Current computer models can faithfully simulate many of the important aspects of the global climate system, such as how global average temperature changes over many decades, the march of the seasons on large spatial scales; and how the climate responds to large-scale forcing, like a large volcanic eruption. So we can be confident that they correctly represent some of the “big picture” features of climate. However, simulations of climate at more regional and local scales, such as a country or state, can still be uncertain. Models also often have difficulty simulating year-to-year changes in the climate system, so a model run in 2014 is unlikely to precisely predict the global temperature in 2015 or 2016.
It is also important to note that projections for this century should not be viewed as predictions. Rather, they represent a range of possible futures, consistent with different concentrations of greenhouse gases in the atmosphere. If we emit a particular level of greenhouse gases in the coming decades, the projection provides us a glimpse of how different our climate might be.
While greenhouse gases continue to accumulate in the atmosphere, the climate will warm. And even if we were able to “stabilize” the concentrations of greenhouse gases in the atmosphere, the planet will continue to warm for many decades, as time lags within the climate system are relatively long.
It can be most useful to think about climate change through a risk management lens – the more greenhouse gases that we emit, the greater the risks for dangerous impacts to occur. Through this lens, reducing emissions helps lower our risks, and the greater the reductions, the greater the risk avoided.
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