Discussing the science associated with climate change can elicit numerous questions. This Q&A document answers some of the most common questions about climate change, its causes, what scientists know, what the future with climate change will look like, and more. All of the answers draw on scientific knowledge about climate change to help you better understand the issue and effectively communicate about it.
This FAQ was updated June 2011.
Is climate change a natural or human-caused phenomenon?
Two completely different lines of evidence demonstrate that the warming of recent decades results primarily from the release of greenhouse gases, chiefly CO2, by human activities. Though present, natural drivers are too weak or are trending in the wrong direction to explain the observed climatic changes.
First, the observed, simultaneous warming of the lower atmosphere (troposphere) and cooling of the upper layers of the atmosphere (stratosphere and higher) in recent decades is a unique ‘fingerprint’ of warming by greenhouse gases. Warming from an increase in solar intensity would warm all layers of the atmosphere simultaneously. Over the past 30 years, satellite observations show that the troposphere has been warming while the stratosphere has been cooling simultaneously (see figure).
A more detailed, state-of-the-art attribution of various climate trends is possible using optimal fingerprinting  approaches that match individual forcings (for example, greenhouse gases, solar intensity or airborne particles) to observed climate change patterns using global climate models. This technique has detected human-induced trends in a wide variety of climate variables including land surface warming, vertical warming of the oceans, loss of Arctic sea ice cover, and changes in precipitation patterns at different latitudes on the Earth. Observations of global land and ocean surface warming and warming of all continents except Antarctica show that no combination of forcings that excludes manmade greenhouse gases can explain the warming trend of the past half-century (see figure).
How do we know greenhouse gases are increasing because of human activity?
Some greenhouse gases (GHG), such as industrial halocarbons, are only made by humans, and thus their presence in the atmosphere can only be explained by human activity.
For naturally occurring GHG, several independent lines of evidence make it crystal clear that they are increasing because of human activities:
Together, all of these independent lines of evidence leave no doubt that GHG concentrations are increasing because of human activities.
What countries are responsible for greenhouse gas emissions and climate change?
Once emitted, GHGs can remain in the atmosphere for 10 years to thousands of years, depending on the gas. This means that emissions that occurred long ago are still in the atmosphere and still affecting the Earth's climate system. Rich industrialized countries have been emitting large quantities of GHGs since the start of the industrial revolution in the mid-18th Century. The United States is responsible for the largest fraction of cumulative CO2 emissions from energy use since 1850 (see figure). Just four countries account for half and 17 countries account for 80 percent of historic energy-related emissions. Only five developing countries rank among the top 20 emitters of cumulative CO2. If we included CO2 emissions from land use change—about 20 percent of human-produced CO2 emissions are from this source—then a few developing countries like Brazil and Indonesia would move up the list but would not be at the top. However, GHG emissions from the most rapidly developing countries in Asia, Africa, and Latin America are catching up with those of the developed world. Indeed, China’s annual GHG emissions already exceed those of the United States and the European Union. Unless future development follows a low-carbon energy path, developing countries will be responsible for most of the growth in GHG concentrations in the future. Determining responsibility for climate change necessitates consideration of these complex patterns of development, past, present, and future.
Has the climate already begun to change and how do we know?
Yes, the climate is changing. Scientists studying this question have arrived at the conclusion that the planet is warming based on millions of thermometer measurements around the world, a global rise in average sea level measured by coastal tide gauges and space satellites, and the loss of snow and ice cover around the world over the past few decades. This evidence is so strong that the National Academy of Sciences concluded , "Climate change is occurring, is very likely caused primarily by the emission of greenhouse gases from human activities, and poses significant risks for a range of human and natural systems."
The Global Change Research Program of the United States found  that impacts from climate change are already occurring across the United States, including more heat waves, more heavy downpours, increased drought in some regions and more flooding in others, and loss of mountain snowpacks, with implications for the security of water supplies. These impacts are occurring sooner and more rapidly than expected and are projected to increase in the future.
Is warming really happening? I’ve heard that weather stations give bad data and that the world is actually cooling.
The facts speak for themselves: As of 2010, the 15 warmest years on record since 1880 all occurred in the last 16 years. Moreover, the 2000s was the warmest decade followed by the 1990s and then the 1980s (see figure). The record warmth of 2010 adds to the huge body of evidence  that the earth continues to warm.
A few skeptics argue that locating most weather stations in urban areas causes an artificial warming trend, but this argument cannot explain why both urban and rural stations as well as satellite data also show warming. Moreover, a recent study  found that the poorly sited stations the skeptics point to actually produce artificial cooling, so if anything, these stations cause warming to be understated, not exaggerated.
Finally, some skeptical scientists recently undertook their own analysis  of surface temperature data and got the same results  that other scientists have reported. The study’s lead, Dr. Richard Muller, recently told Congress :
“In our preliminary analysis of these stations, we found a warming trend that is shown in the figure. It is very similar to that reported by the prior groups: a rise of about 0.7 degrees C since 1957.”
How do you explain seasonal cold weather and heavy snowfall if the climate is warming?
Global warming is about changes in long-term averages and not about single events; it does not mean an end to cold weather. Instead, it means that cold weather will become less frequent and hot weather more frequent when averaged over decades. In fact, both of these trends have been observed over the past 50 years in the United States and globally. Even with global warming, we will have cold winters; however, there will be fewer of them. It is also important to remember that a cold winter for one location doesn’t mean a cold winter everywhere. In fact, many parts of the world, including the Arctic and the tropics, had an unusually warm winter in 2010.
To create heavy snowfall the East Coast experienced during the 2009 and 2010 winters, you need two things: moist air and cold air. In recent winters, the Gulf of Mexico and the tropical Pacific have supplied lots of moist air, and that is the key to getting heavy precipitation. We also had more cold air than usual that spilled out of the Arctic. Conditions were just right in the past two winters for these air masses to meet up and create massive snowstorms. Snowfall occurs when warm, moist air is forced above the cold air and begins to precipitate into the cold air, causing what would haven rain to freeze. Since climate change increases the moisture content of the atmosphere, global warming can actually increase the risk of heavy snowfall.
What are the current projections for 21st century climate change?
The Intergovernmental Panel on Climate Change’s (IPCC) most recent projections for 21st century average global temperature increase is 2-11°F, based on the spread across a large number of climate models and assumptions about 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 percent higher than the global average. The Arctic is likely to experience the greatest warming and has already been warming at twice the global rate.
Recent sea level rise studies have projected that water levels could rise between 1.5 and 6 feet by the end of this century, depending on the rate and magnitude of future 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 are likely to be very unevenly distributed with wet areas continuing to get wetter while dry regions may experience a decrease in precipitation.
What harm will climate change cause?
If we continue on the current path, U.S. coastal areas will be at increased risk from sea level rise and more intense storms that erode beaches and damage structures; southern states will be at risk from increased droughts, floods and heat waves; Midwestern states will face more extreme flooding and more heat waves; and western states will face increased droughts, invasive species, and wild fires.
In addition, scientists point to more rapid sea level rise, increased loss of ice from land-based glaciers and ice sheets, and the dramatic loss of Arctic sea ice. Sea level rise will increase coastal flooding and eventually inundate low-lying areas, including many coastal cities around the world. Earlier spring snowmelt will also reduce the clean water supplies on which many parts of the world depend.
These climate impacts are likely to be costly. We can get a sense of the magnitude of these costs by looking at examples of the type of severe weather events that are likely to get worse because of climate change, such as Hurricane Katrina, the 2010 Russian heat wave, and Nashville’s “1000-year” flood in 2010. The damage from Hurricane Katrina was equivalent to one-third of the combined gross domestic product of Louisiana and Mississippi. In New Orleans, property damage excluding reductions in economic output exceeded $80 billion. The city’s population was cut in half as 200,000 people fled. Today, the population is only at 88 percent of pre-Katrina levels.
Climate change will not just harm property, though. Human health may be affected by climate change through extreme heat waves, an exacerbation of air pollution, severe weather, and increased spread of infectious diseases.
Is there a link between extreme weather and climate change? What can we learn about climate change from extreme weather?
Extreme weather events, especially heat waves and heavy precipitation (both rain and snow), have been getting more frequent and more intense for several decades. However, when talking about climate change, it is critical to understand that the change in the average of all the events combined is what defines climate change. Consequently, it makes little sense to say that climate change did or did not “cause” a particular weather event. It is accurate, however, to say that climate change increases the risk  of extreme weather events occurring.
Even though we cannot assign a “cause” to a particular event, there is a lot we can learn by studying how actual events impact local systems, human wellbeing, and economies. Extreme weather events teach us about the risks we face from climate change and our current vulnerabilities to future climate change events. Examples such as Hurricane Katrina in 2005, the 2010 Russian heat wave, and Nashville’s “1000-year” flood in 2010 tell us about how we are able to handle similar extreme weather events and what we need to do to prepare for more events like those in the future.
Won’t some warming benefit countries that have a lot of cold weather?
Some people argue that cold countries are likely to be climate change “winners .” For example, warmer temperatures in Russia could reduce heating fuel consumption, lengthen the agricultural growing season, and open up transportation routes and access to mineral and energy deposits in the Arctic. But these types of analyses inevitably focus on a few simplistic variables, while neglecting a plethora of more complex and likely negative impacts. Consider the many negative effects of the extreme heat wave Russia experienced in summer 2010. That single event destroyed a third of Russia’s wheat crop, prompting Russia to suspend grain exports, which caused food prices to rise globally. The heat wave killed 15,000 people and shaved $123 billion off Russia’s GDP. Results of a recent peer-reviewed scientific study  “suggest that we may be on the cusp of a period in which the probability of such events increases rapidly, due primarily to the influence of projected increases in greenhouse gas concentrations.” If these events do become common in future decades, it is hard to see Russia being a climate-change winner.
What is the greenhouse effect?
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 60oF colder.
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 slow the escape of that heat into space, raising the temperature of the atmosphere and the earth's surface. The greenhouse effect is natural, but human-induced increases in greenhouse gas concentrations further slow the escape of heat, causing additional warming, which changes the climate.
What are the most important greenhouse gases and their sources?
The list below of greenhouse gases is in order of most abundant in the atmosphere.
Aerosols– While not a gas per se, 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 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, especially over Antarctica where the “ozone hole” exists in the upper atmosphere. 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. The ozone hole does not contribute to global warming by letter more of the sun’s rays reach the Earth.
Ozone in the lower atmosphere is an air pollutant that contributes to health problems. Ozone in the lower atmosphere has increased over the past century because of increasing automobile and industrial pollution, thus contributing to global warming.
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, converting that carbon to wood and locking it away from the atmosphere. Keeping forests intact instead of burning them or cutting them down can prevent CO2 from being released and expanding forests can enhance removal of CO2 from the atmosphere.
What role do black carbon aerosols (also known as soot) play in global climate change?
Black carbon  aerosols or soot are tiny, 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 dark-colored 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, when the black particles settle on snow or ice, they accelerate melting. Consequently, black carbon is especially effective at accelerating warming in snowy places, like the Arctic and snow-capped mountains. This raises concerns about the loss of snowpacks and glaciers that supply man people with water, such as in South and East Asia where the Himalayan glaciers supply major rivers with water.
Black carbon originates mostly from diesel engines in trucks and ships, burning coal in stoves, burning wood and dung for fuel, and forest fires. Many of these activities occur predominantly in developing countries. One of the major concerns about black carbon is that it causes health problems in local population. Reducing black carbon emissions would therefore improve public health and limit climate change.
Are scientists in agreement about the reality and cause of climate change?
A recent poll  of Earth scientists found that, regardless of their specialty, 82 percent of agreed that the climate is warming and that human activities are contributing to this trend. When specialties are considered, 97 percent of scientists who specifically study climate system agree that recent warming is real and is almost certainly caused by human activities. Uncertainty remains about how particular aspects of climate change will play out in the future, such as changes in cloud cover or the timing or magnitude of droughts and floods. These do not reflect uncertainty about the basic fact that the climate is changing or that human activities are the primary cause.
How do scientists project 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:
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 more useful way to think about climate models is not as a specific prediction, but rather as a range of possible futures. Most of the current models do a reasonable job of simulating past climate variability on time scales of decades to centuries, but such models are not designed to predict short-term climate variability (days-years) and weather. (Weather forecasters use a different kind of model that can predict the weather but it not designed to project beyond 10 days.) Much of the uncertainty in future projections is due to the assumptions used regarding future trends in greenhouse gas emissions, which is a question of human behavior, not climate science.
Did “Climategate” undermine any of the findings of climate change science?
In November 2009, more than 1,000 emails belonging to the Climatic Research Unit (CRU) at the University of East Anglia in the United Kingdom were disclosed without authorization by an unknown party. The contents of a relatively small number of the email messages became the basis for the controversy commonly known as “Climategate.” The scientists involved were cleared of any scientific misconduct by six independent investigations  in the U.K. and the United States.
From these investigations, we learned that there is no evidence of scientific misconduct by any of the scientists, and more importantly, nothing emerged from the investigations that alters our understanding of the science. Although a small percentage of the emails were impolite and some expressed animosity toward climate change skeptics, accusations of misconduct levied against the authors of the emails lacked merit. Moreover, the scientific data referenced in the emails are not essential to our understanding of climate change, and those data that are essential were not involved in the controversy. After a full airing of this matter, it remains clear that the scientific consensus  on climate change, as stated by the IPCC, the U.S. National Academy of Sciences, and virtually every relevant American scientific society, remains unaltered.
Is the science too uncertain to justify taking action now?
When it comes to climate change, uncertainty is not a reasonable barrier to action. Indeed, the very fact that it is difficult to pinpoint exactly how climate change will play out in the future is ample motivation to take action to limit its unpredictability by limiting its extent. Too much is at stake to risk inaction.
There is strong scientific consensus that the warming of the climate system is unequivocal; warming since 1950 is very likely due to manmade greenhouse gases (GHGs) accumulating in the atmosphere; and unabated emissions of GHGs will very likely cause further warming in the range of 2-11°F by 2100. This wide range of projected temperatures is a careful expression of scientific uncertainty. Hence, uncertainty doesn’t mean we know nothing; just that we do not know precisely what the future may hold in a given place at a given time. But scientific uncertainty helps inform what the risks of climate change look like. Will the oceans rise by two feet or six? Will global average temperatures rise by two degrees or five?
The things that scientists are uncertain about, such as the timing and magnitude of future change in the climate, do not cast doubt on what scientists are certain about. What we do know about the science of climate change tells us we need to act now to manage the risks associated with it. Moreover, what we do not know CAN hurt us, and that is the best reason to limit how far much the climate will change in the future and to take measures to become more resilient to the changes that are already unavoidable.
To what extent can humans adapt to climate change?
Some degree of adaptation will undoubtedly be necessary to respond to climate change that is already 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. Poor countries and poor populations within rich countries will be disproportionately impacted by climate change because of their limited resources for adaptation.
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. By investing in the reduction of greenhouse gases, it will offset necessary investments in adaptation in addition to protecting against those adverse effects of climate change for which adaptation is particularly difficult. It isclear that responding to climate change requires both mitigation of greenhouse gases and adaptation to unavoidable change.
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 gas concentrations 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. A recent report  from the National Academy of Sciences, to stabilize the atmospheric CO2 concentration at the current level would require emissions to peak now and be reduced by more than 80 by the end of this century. Obviously, emissions cannot peak immediately, so it will not be possible to stabilize CO2 at the current level. The longer emissions reductions are delayed, the higher the concentration that can be stabilized.
Is planting more trees a way of solving global warming?
Increasing the world's forest cover is a useful mechanism for mitigating atmospheric CO2 concentrations because of the ability for plants to remove CO2 from the atmosphere through photosynthesis. However, not all forests are equal in this regard. Northern forests are generally re-growing today after having been largely cut in the 19th and early 20th centuries, so there is less opportunity to use these forests for additional mitigation. Tropical forests are by far the most important because they suck up the most CO2 every year and because they are intentionally being burned and cut down at a rapid pace. Tropical deforestation contributes about 20 percent of the human-produced CO2 each year, so reducing this deforestation effectively reduce CO2 emissions. However, even a vigorous global reforestation program would not be sufficient to offset all anthropogenic CO2 emissions from human sources. 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.