Global warming is caused by the buildup of heat-trapping gases in the atmosphere. These gases come primarily from the burning of coal, oil and natural gas, with additional contributions from the cutting and burning of forests. The human-induced increase in these gases has caused the Earth to warm by about 1.5ºF over the past century.

Climate change is increasing the frequency and intensity of some types of extreme weather. For example, warming has caused more rain to come in heavy downpours. There are longer dry periods between rainfalls. This, coupled with more evaporation due to higher temperatures, intensifies drought. Heat waves are becoming more frequent and intense, while cold extremes have decreased.

It is true that 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. 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. However, recent climate changes are inconsistent with trends caused by natural forces. Though some of the warming in the first half of the 20th century was likely due to an increase in solar output, recent changes due to such naturally occurring factors are inconsistent with observed warming. In fact, without human influences, Earth’s climate actually would have cooled slightly over the past 50 years. 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.

Climate changes are already being observed around the United States. The average U.S. temperature has increased more than 2F over the past 50 years. This warming has generally resulted in longer warm seasons and shorter cold seasons. Temperatures are projected to continue rising; how much they rise depends primarily on the amount of heat-trapping gases emitted globally. The interior of the continent is generally projected to see greater temperature increases than coastal areas.

While precipitation over the United States as a whole has increased, there have been important regional differences. Wetter areas, such as the Northeast, have generally become wetter, while drier areas, such as the Southeast, have become drier. This fits the pattern projected to occur due to global warming. There have also been important seasonal differences, with some seasons showing large increases or decreases in precipitation in various regions.

The Northeast and Midwest have seen large increases in heavy downpours over the past 50 years. Parts of the Southwest and Southeast have seen increasing droughts. The West has generally seen declining snowpack due to rising temperatures, meaning that less water is available in summer and fall, and reservoir levels are down.

Future changes in precipitation are harder to project than changes in temperature, though models generally suggest a continuation of observed patterns, with northern areas receiving more precipitation, particularly in winter and spring, and southern areas seeing less precipitation in those seasons. This is because the interaction between warm, moist air coming from the south and colder air coming from the north will occur farther north than it did on average in the last century.

Climate models have long predicted that global warming would affect the Arctic faster than the rest of the world. Arctic temperatures have risen dramatically in recent decades, with one of the results being a decline in the sea ice that covers much of the Arctic Ocean. Adding to the pace of warming in the Arctic is the fact that as sea ice melts, it reveals darker water beneath, which absorbs more of the Sun’s heat, causing more warming, and hence more melting, in a self-reinforcing cycle. Scientists project that summer sea ice in the Arctic could disappear entirely in the coming decades. Because sea ice affects reflectivity, cloudiness, humidity, exchanges of heat and moisture at the ocean surface, and ocean currents, the loss of Arctic sea ice has significant climatic ramifications that extend beyond the Arctic region. A helpful metaphor is to think of the Arctic as an “air conditioner” for the planet. As Arctic ice disappears, the planet heats up faster.

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 orbital tilt 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 is 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 warm episodes may have been initiated by orbital changes that caused warming and thus caused CO2 to rise, which then led to more warming, the current warm episode is being driven by increasing CO2 due primarily to fossil fuel burning.

Global warming cannot be stopped immediately, but its rate can be slowed, and temperatures can be stabilized by late this century, if there are large reductions in heat-trapping pollution. Global temperature has already increased about 1.5F and another degree or so of warming is already “in the pipeline” given past emissions. So the choice we face is whether we will experience about another 2F in this century, or twice or three or four times that much. Global emissions will be the primary determinant of the outcome. A wide variety of impacts become much more severe at higher levels of warming.

While some media outlets carried this story back in the 1970s, most climate scientists at the time were not concerned about global cooling. Though climate science was less developed at that time, the vast majority of climate science papers published were related to the same concern that prevails today: warming due to the increase in heat-trapping gases. There were a few papers published on the issue of particle pollution blocking out some of the incoming sunlight, an issue still with us today. And a few articles related to this in the popular press carried dramatic headlines. A recent paper in Bulletin of the American Meteorological Society examined this issue in detail and concluded: “There was no scientific consensus in the 1970s that the Earth was headed into an imminent ice age. Indeed the possibility of anthropogenic warming dominated the peer-reviewed literature even then.”