Climate Science Q&A

What’s the difference between “global warming” and “climate change”?

“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 includes many other impacts resulting from global warming (for example, changes in precipitation and ocean acidity) and different geographic scales (for example, a continent, an ocean, a hemisphere, the planet).

Are scientists in agreement about the reality and cause of climate change?

Yes. Climate scientists overwhelmingly agree that human activities are the primary cause of climate change. For example, this study shows that 97 percent of science papers addressing climate change support the consensus that humans are causing global warming, which is consistent with surveys of climate scientists and other peer-reviewed studies.

Many aspects of climate change continue to be studied to reduce the uncertainty in and improve the usefulness of climate projections. How will increased ice sheet melt affect ocean currents, and vice versa? How will the future distribution, altitude, and water content of clouds affect future temperatures? Studying these questions to determine the range in climate change projections helps decision-makers evaluate risk and plan for the future. Despite some inherent uncertainty in climate science, there is high confidence and agreement that the planet is warming, and that human activities are the primary cause.


Is climate change a natural or human-caused phenomenon?

Human activities that release carbon dioxide and other greenhouse gases into the atmosphere are largely responsible for the climate change observed over the last century. 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 the addition of greenhouse gases to the atmosphere, and inconsistent with other factors that can affect the global temperature over many decades, like changes in the sun’s energy.

While it is true that the climate has changed throughout all of Earth’s history as a result of natural forces (like volcanic eruptions and variations in the sun’s energy), the current situation is very different. Natural forces alone cannot account for the warming that has occurred, and the pace of warming is unique in Earth history.

The congressionally-mandated National Climate Assessment summarizes the state of our knowledge: 

Greenhouse gas emissions from human activities are the only factors that can account for the observed warming over the last century; there are no credible alternative human or natural explanations supported by the observational evidence. Without human activities, the influence of natural factors alone would actually have had a slight cooling effect on global climate over the last 50 years.

How much warmer will the Earth get?

Projections for the likely increase in average global temperature this century range from about 4°F (2° C) to 9°F (5° C) warmer than preindustrial temperatures (Ch 2, Section 2, NCA).

The large range among projections stems mostly from uncertainty about future greenhouse gas emissions. To minimize warming , significant cuts in emissions need to be implemented starting now (e.g. the Rapid Emissions Reductions scenario in the figure below). Recent greenhouse gas emissions and current stated policies surrounding future emissions correspond much more closely to the high end of the warming projections.

However, this warming will not be spread evenly around the globe. At the higher latitudes, warming is likely to be more than the global average (see figure), which is consistent with current warming trends. Since 1901 the average global temperature has increased by about 1.8F (Ch 2, Section1, NCA), while the Arctic has warmed more than twice as quickly due to local feedbacks like the loss of sea ice, glaciers, and snow cover.

Increases in the average global temperature will also exacerbate heatwaves locally, making extremely hot days more common and more dangerous.

Projected Change in Average Annual Temperature

Will we still have winter?

Climate change does not mean an end to cold weather. Instead, averaged over many decades, cold winters and mild summers will become less frequent, and mild winters and hot summers will be more frequent. Similarly, a baseball batter who takes steroids has a higher chance of hitting more home runs in a season than they did before taking steroids. Just as the batter can still strike out occasionally, the climate system has more extremely hot days over decades, but can still produce very cold days occasionally.

In fact, these trends have been observed.  In the first half of the 20th century, there was an even split between the number of record cold days and record hot days. In recent decades , though, hot records continue to be broken while cold records almost never are. It is very likely that the number of cold days and nights has decreased and the number of warm days and nights has increased on the global scale (IPCC AR5, Synthesis for Policymakers).

It is also important to remember that a cold winter in one location can still happen while the global average temperature is increasing. A cold winter in one place doesn’t mean a cold winter everywhere.

Won’t some parts of the world benefit from climate change?

There are some benefits that come with warming and higher atmospheric carbon dioxide levels:

  • Energy demands for heating usually decrease
  • Carbon dioxide can accelerate growth in some types of crops
  • Growing seasons get longer, which may increase agricultural production
  • Illness and mortality related to cold decline

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 has more downsides than benefits.

According to the National Climate Assessment and the IPCC Assessment 2014 Report: Impacts, Adaptation, and Vulnerability, potential harm to individuals, communities, and businesses includes threats to:

  • Coasts – In the near-term, sea-level rise increases storm surge, making hurricanes and other severe storms more destructive. It may also contaminate groundwater supplies with saltwater. In the longer term, many coastal communities may become inundated, forcing choices about investing in shoreline protection and/or moving farther inland. For people who live in small island nations, where higher ground may be limited, resettlement in new countries may be necessary.
  • Water resources – Water may become less available because of changes in precipitation patterns, loss of snowpack, and earlier snowmelt. Warmer temperatures can drive up water demands for agriculture, energy, and human consumption. Flooding from heavier rainfall events can also potentially overcome wastewater treatment systems and spread agricultural runoff into water bodies.
  • Health – Warmer temperatures can increase the risks of heat-related illness and even death. Warmer temperatures can also help expand the ranges of diseases carried by insects or ticks, bringing them to regions where they were previously not a threat. Warmer temperatures can increase smog, reducing air quality and causing health issues for the young, elderly, and those with respiratory problems.
  • Security – Climate change can affect access to basic needs (food, water, energy, shelter), especially in developing countries. Impacts on these critical resources can trigger or exacerbate migration, conflict, and political instability, which have security implications for the United States. In addition, loss of Arctic sea ice presents new operational issues for the U.S. Navy and for the security of our Arctic border.

How reliable are climate projections?

Resources on climate models and projections can be found at the National Academy of Sciences. They also have a helpful interactive graphic that shows how climate models are created, tested, and used to determine how our climate will continue to change in the future.

Current computer models can faithfully simulate many of the important aspects of the global climate system, such as changes in global average temperature over many decades; the march of the seasons on large spatial scales; and how the climate responds to large-scale forcing, like a major volcanic eruption. We can be confident that climate models correctly represent some of the “big picture” features of climate, though simulations of climate at more regional and local scales have greater uncertainty. However, the largest source of uncertainty in our future climate beyond the next few decades is future greenhouse gas emissions.

The models used to simulate our future climate use different greenhouse gas emissions scenarios to reveal a range of possible future climates. If we emit a particular level of greenhouse gases in the coming decades, the projection best matching these emissions provides us a glimpse of how different our climate is likely to be.

How much do greenhouse gas emissions have to be reduced to stop climate change?

According to the IPCC, we need to keep global warming to under 1.5°C to avoid most of the dangerous impacts of warming. In order to hit this target, annual emissions need to be reduced, and by the end of the century there needs to be net removal of greenhouse gases from the atmosphere. The longer we wait to reduce greenhouse gas emissions, the more drastic reductions we will need to make to minimize warming.

While greenhouse gases continue to accumulate in the atmosphere, the climate will warm. And even if we stabilized 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.