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Q Is Earth’s climate warming?

Yes, it is an unequivocal fact that, since the early 20th century, Earth’s average temperature has risen and continues to rise, despite some natural year-to-year fluctuations. Each of the past few decades has been substantially warmer than the decade prior to it. The hottest five years on record are 2014-2018.

All analyses of all surface temperature data sets compiled by major climate centers around the world show a clear warming trend. Besides these thousands of thermometer readings from weather stations around the world, there are many other clear indicators of global warming such as rising ocean temperatures, sea level, and atmospheric humidity, and declining snow cover, glacier mass, and sea ice.

Because temperatures vary from year to year, scientists measure trends in running averages and analyze trends over decades rather than expecting every year to be hotter than the previous year. Some years have particular factors that make them hotter than those just before and after. For example, a major El Niño event combined with the persistent rise in heat-trapping gases made 1998 one of the hottest years on record. That has caused some people to claim that Earth has been “cooling” since then. But as the data clearly show, this claim is false.

Data from NOAA Global Temperature from 1880 to 2018

A graph showing a steady increase in global land and ocean temperatures since 1880

From NASA Climate Change Temperature map from 1880 to 2018

This video shows a time lapse of the temperature anomalies of different parts of the Earth between the years 1880 and 2018. Over time, the map shows increasingly more anomalous temperatures, especially in the northern hemisphere.

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Q Do climate scientists agree that the world is warming and that humans are the cause?

More than 90% of climate scientists have concluded that human-caused global warming is happening. It is well-established that human activity is the dominant cause of the warming experienced over the past 50 years. This conclusion is based on multiple lines of evidence, from basic physics to the patterns of climate change through the layers of the atmosphere. The warming of global climate and its causes are not matters of opinion, they are matters of scientific evidence, and that evidence is clear.

These two basic conclusions, that the world is warming and that humanity is the primary cause, are well-documented in the reports of the Nobel prize-winning Intergovernmental Panel on Climate Change. Further, these major conclusions have been objectively reviewed and independently verified by the National Academies of Sciences of all major countries including the U.S., and all relevant scientific organizations such as the American Geophysical Union, American Meteorological Society, American Association for the Advancement of Science, and the Royal Society of the United Kingdom.

From It’s Okay To Be Smart More than 90% of Climate Scientists Really Do Agree

Do 97% of climate scientists really agree that humans are the main cause of climate change? Yep! Here’s what the 97 percent statistic really means.

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Q How do we know recent climate change is caused by human factors rather than natural factors?

Climate changes observed over recent decades are inconsistent with trends caused by natural forces but are totally consistent with the increase in human-induced heat-trapping gases. In fact, without human influences, Earth’s climate actually would have cooled slightly over the past 50 years.

Natural forces cause Earth’s temperature to fluctuate on long timescales due to slow changes in the planet’s orbit and tilt. Such forces were responsible for the ice ages. Other natural forces sometimes cause temperatures to change on short timescales. For example, major volcanic eruptions can cause short-term cooling lasting two to three years. Changes in the sun’s output over the past 30 years have followed the typical 11-year cycle, with no net increase, while temperatures were warming strongly.

Many independent lines of evidence (from basic physics to the patterns of temperature change through the layers of the atmosphere) have shown that the warming of the past 50 years is primarily due to the human-caused increase in heat-trapping gases.

From NASA/GSFC GISS Natural vs. Manmade Causes Compared

When you compare possible natural causes (orbital, solar and volcanic activity) over the course of the last 100+ years, to possible manmade causes (aerosols, deforestation and greenhouses gases), the winner is clear. Greenhouse gases directly correlate to an increase in Earth temperature.

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Q What do ice cores tell us about the relationship between temperature and carbon dioxide?

We know from ice core records that temperature and carbon dioxide (CO2) levels are closely correlated. In the distant past, warming episodes appear to have been initiated by cyclical changes in Earth’s orbit around the sun that caused more summer sunlight to fall in the northern hemisphere. This caused snow and ice on land and sea to melt, revealing darker land and water, which caused more warming, in a self-reinforcing cycle. As the planet continued to warm, more CO2 was released from the oceans, and this increase in heat-trapping gas caused even more warming. Thus, while CO2 did not initiate those warming episodes, it did contribute to them.

In the current warming episode, it is clear that CO2 and other human-induced heat-trapping gases are driving the warming. We know with certainty that the increase in CO2 concentrations since the Industrial Revolution was caused by human activities because the isotopes of carbon show that it comes from fossil fuel burning and the clearing of forests.

So even though past warming episodes may have been initiated by orbital changes that caused warming and thus caused CO2 to rise, which then led to more warming, we know that the current warming episode is being driven by increasing CO2 due to the burning of fossil fuels and the clearing of forests. The orbital changes that caused the ice ages are far too weak and slow to cause a warming as rapid as the current one.

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Q How can we trust predictions about our climate for decades or centuries in the future?

When people wonder about the degree of certainty in global warming predictions versus next week’s weather report, they are confusing climate and weather. Predicting weather and predicting climate are different and pose different challenges.

Weather is individual, day-to-day atmospheric events; climate is the statistical average of those events. Weather is short-term and chaotic and is thus inherently unpredictable beyond a few days. Climate is long-term average weather and is controlled by larger forces, such as the composition of the atmosphere, and is thus more predictable on longer timescales. For the same reasons, a cold winter in one region does not disprove global warming.

As an analogy, while it is impossible to predict the age at which any particular man will die, we can say with high confidence that the average age of death for men in industrialized countries is about 75. The individual is analogous to weather, whereas the statistical average is analogous to climate.

From National Geographic Channel Weather Versus Climate Change

Neil deGrasse Tyson breaks down the differences between weather and climate change.

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Q How reliable are climate models?

Climate models are mathematical representations of the interactions between the various aspects of the climate system including the atmosphere, oceans, land surface, ice, and the sun. The complex task of simulating Earth’s climate is carried out by computer programs designed to detect long-term climate trends based on large-scale forces. Unlike weather prediction models, climate models are not intended to predict individual storm systems.

Climate models are tested against what we know happened in the past and they do accurately map past climate changes. Climate models have also been proven to make accurate predictions. For example, the eruption of Mt. Pinatubo provided an opportunity for such a test. The models successfully predicted the climatic response after the eruption, a cooling influence that lasted a couple of years.

Models have also been applied to the question of how the climate system will react to additional greenhouse gases. These models have correctly predicted effects subsequently confirmed by observation, including greater warming in the Arctic and over land, greater warming at night, and stratospheric cooling.

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Q What did most climate scientists in the 1970s predict about future climate?

The vast majority of published climate science papers in the 1970s were related to the same concern that prevails today: warming due to the increase in heat-trapping gases. There were a few papers published at that time on the issue of particle pollution (mostly from coal plants which did not yet have scrubbers) blocking out some of the incoming sunlight and exerting a short-term cooling influence. Some media outlets picked up on this and sensationalized the notion of global cooling, contrary to the concerns of most climate scientists.

Because it has been a persistent myth that scientists warned of cooling in the 1970s, researchers examined this question and published their findings in 2008 in the Bulletin of the American Meteorological Society. They concluded: “There was no scientific consensus in the 1970s that the earth was headed into an imminent ice age. Indeed the possibility of anthropogenic [human-caused] warming dominated the peer-reviewed literature even then.”

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Q How do we know recent warming is NOT caused by the sun?

Since 1978, scientists have been using sensors on satellites to measure the amount of the sun’s energy reaching the top of Earth’s atmosphere. Since that time, global temperatures have risen sharply, while there has been no significant change in the amount of the sun’s energy reaching Earth.

In addition, if the warming had been caused by an increase in the sun’s energy, we would expect to see warming throughout the layers of the atmosphere, from the surface all the way up through the stratosphere. On the other hand, warming caused by a buildup of heat-trapping gases from human activities would cause warming at the surface but cooling in the stratosphere, and this is in fact what we observe.

From Climate Communication How We Know Climate Change Isn't Caused by the Sun

A Professor of Geosciences and Astmospheric Sciences from the University of Arizona explains why climate change isn't caused by temperature variations in the sun.

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Q Do warmer cities affect the global temperature record?

No. The “urban heat island” effect is undoubtedly a real phenomenon that has been recorded in major cities around the world. It results from the large amounts of concrete and asphalt in cities absorbing and holding heat and the minimal amount of vegetation to provide shade and evaporative cooling. However, scientists have accounted for these local effects and have verified that they do not skew the global temperature record. For example, one test scientists have done is to remove all the urban stations from the global temperature record. When this is done, the global warming of the past 50 years is still apparent.

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Q How do volcanoes influence climate?

Volcanoes can and do influence global climate, exerting a cooling influence for a few years. This cooling influence occurs when large, explosive volcanic eruptions inject sun-reflecting sulfate particles into the high reaches of the atmosphere (the stratosphere). For example, the four major volcanic eruptions of the 20th century caused short-term interruptions in the long-term warming trend caused by human induced emissions of heat-trapping gases.

Contrarians have asked whether the CO2 emissions from volcanoes might impact climate. But in fact, this is insignificant compared to human activities. Burning fossil fuels releases several hundred times more CO2 than volcanoes do each year. Fossil fuel burning results in the emission of approximately 35 gigatons of CO2 into the atmosphere per year worldwide. This obviously dwarfs the estimated annual release of CO2 from volcanoes, which is 0.15 to 0.26 gigatons per year.

From Climate Communication The four major volcanic eruptions of the 20th century caused short-term interruptions in the long-term warming trend

A graph showing four volcanic eruptions since 1900 plotted against global temperature anomalies.
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