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. Polls of climate scientists show there is no “debate” within the field. For example, this study shows that 97 percent of science papers addressing climate change support the consensus that humans are causing global warming.

Many issues related to climate change require further study. How fast will the ice sheets melt? How are changes in the jet stream related to climate change? But scientists agree 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 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.

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:

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. 

How much warmer will the Earth get?

Projections for the likely increase in average global temperature 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 uncertainty about future energy use and greenhouse gas emissions. To keep warming to the lower end of the range, significant cuts in emissions would need to be implemented starting now. Greenhouse gas emissions trends in recent decades correspond much more closely to the high end of the warming projections.

Projected Change in Average Annual Temperature

More recent analysis suggests an even grimmer picture. A 2017 study finds that without significant and immediate action to reduce emissions, there is only a 5 percent chance that warming by 2100 will be less than 2°C.

Will we still have winter?

Climate change does not mean an end to cold weather. 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. Extreme temperature records globally have changed. 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 years, though, hot records continue to be broken while cold records almost never are. It is also important to remember that a cold winter for one location doesn’t mean a cold winter everywhere.

Won’t some parts of the world benefit from warmer weather?

There are some benefits that come with warming and increased carbon dioxide:

  • 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 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:

  • Coasts – In the near-term, sea-level rise is likely to increase 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, or 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 projections and climate models can be found at the National Academy of Sciences. They also have a helpful interactive graphic.

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 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 can still be uncertain.

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.

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

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.