A thin section of an ice core extracted from under the Greenland Ice Sheet. Credit: NASA Goddard Space Flight Center/Ludovic Brucker. Ice cores are scientists’ best source for historical climate data. Every winter, some snow coating Arctic and Antarctic ice sheets is left behind and compressed into a layer of ice. By extracting cylinders of ice from sheets thousands of meters thick, scientists can analyze dust, ash, pollen and bubbles of atmospheric gas trapped inside. The deepest discovered ice cores are an estimated 800,000 years old. The particles trapped inside give scientists clues about volcanic eruptions, desert extent and forest fires. The presence of certain ions indicates past ocean activity, levels of sea ice and even the intensity of the sun. The bubbles can be released to reveal the make-up of the ancient atmosphere, including greenhouse gas levels. Other tools for learning about Earth’s ancient atmosphere include growth rings in trees, which keep a rough record of each growing season’s temperature, moisture and cloudiness going back about 2,000 years. Corals also form growth rings that provide information about temperature and nutrients in the tropical ocean. Credit: roundstripe/Shutterstock.com

Mount Pinatubo during its 1991 eruption Overall, volcanoes release about 5 percent of the equivalent amount of CO2 released by humans. Quite small. However, about once every 20 years there is a volcanic eruption (e.g., Mount Pinatubo, El Chichon) that throws out a tremendous amount of particles and other gases. These will effectively shield us enough from the sun to lead to a period of global cooling. The particles and gases typically dissipate after about 2 years, but the effect is nearly global.

The greenhouse effect is the way in which heat is trapped close to the surface of the Earth by “greenhouse gases”. These heat-trapping gases can be thought of as a blanket wrapped around the Earth, which keeps it toastier than it would be without them. Greenhouse gases include carbon dioxide, methane and nitrous oxides. Greenhouse gases arise naturally, and are part of the make-up of our atmosphere. Earth is sometimes called the “Goldilocks” planet – it’s not too hot, not too cold, and the conditions are just right to allow life, including us, to flourish. Part of what makes Earth so amenable is the naturally-arising greenhouse effect, which keeps the planet at a friendly 15 °C (59 °F) on average. But in the last century or so, humans have been interfering with the energy balance of the planet, mainly through the burning of fossil fuels that give off additional carbon dioxide into the air. The level of carbon dioxide in Earth’s atmosphere has been rising consistently for decades and traps extra heat near the surface of the Earth, causing temperatures to rise. Credit: NASA Jet Propulsion Laboratory.

Humans have caused major climate changes to happen already, and we have set in motion more changes still. Even if we stopped emitting greenhouse gases today, global warming would continue to happen for at least several more decades if not centuries. That’s because it takes a while for the planet (for example, the oceans) to respond, and because carbon dioxide – the predominant heat-trapping gas – lingers in the atmosphere for hundreds of years. There is a time lag between what we do and when we feel it. In the absence of major action to reduce emissions, global temperature is on track to rise by an average of 6 °C (10.8 °F), according to the latest estimates. Some scientists argue a “global disaster” is already unfolding at the poles of the planet; the Arctic, for example, may be ice-free in the summer within just a few years. Yet other experts are concerned about Earth passing one or more “tipping points” – abrupt, perhaps irreversible changes that tip our climate into a new state. Thwaites Glacier. Credit: NASA. But it may not be too late to avoid or limit some of the worst effects of climate change. Responding to climate change will involve a two-tier approach: 1) “mitigation” – reducing the flow of greenhouse gases into the atmosphere; and 2) “adaptation” – learning to live with, and adapt to, the climate change that has already been set in motion. The key question is: what will our emissions of carbon dioxide and other pollutants be in the years to come? Recycling and driving more fuel-efficient cars are examples of important behavioral change that will help, but they will not be enough. Because climate change is a truly global, complex problem with economic, social, political and moral ramifications, the solution will require both a globally-coordinated response (such as international policies and agreements between countries, a push to cleaner forms of energy) and local efforts on the city- and regional-level (for example, public transport upgrades, energy efficiency improvements, sustainable city planning, etc.). It’s up to us what happens next.

False-color view of total ozone over the Antarctic pole. Purple and blue represent areas where there is the least ozone, yellows and reds where there is more ozone. Credit: NASA Ozone Hole Watch. Yes and no. The ozone hole is basically a man-made hole in the ozone layer above the South Pole during the Southern Hemisphere’s spring. The ozone layer, which lies high up in the atmosphere, shields us from harmful ultraviolet (UV) rays that come from the sun. Unfortunately we punched a hole in it, through the use of gases like chlorofluorocarbons (CFCs) in spray cans and refrigerants, which break down ozone molecules in the upper atmosphere. While some of the sun’s UV rays slip through the hole, Yes and no. The ozone hole is basically a man-made hole in the ozone layer above the South Pole during the Southern Hemisphere’s spring. The ozone layer, which lies high up in the atmosphere, shields us from harmful ultraviolet (UV) rays that come from the sun. Unfortunately we punched a hole in it, through the use of gases like chlorofluorocarbons (CFCs) in spray cans and refrigerants, which break down ozone molecules in the upper atmosphere. While some of the sun’s UV rays slip through the hole, they account for less than one percent of the sun’s energy. So these UV rays cannot explain the global warming of the planet. What scientists have uncovered recently, however, is that the ozone hole has been affecting climate in the Southern Hemisphere. That’s because ozone is also a powerful greenhouse gas, and destroying it has made the stratosphere (the second layer of the atmosphere going upwards) over the Southern Hemisphere colder. The colder stratosphere has resulted in faster winds near the pole, which somewhat surprisingly can have impacts all the way to the equator, affecting tropical circulation and rainfall at lower latitudes. The ozone hole is not causing global warming, but it is affecting atmospheric circulation.

Global temperature rise from 1880 to 2015. Higher-than-normal temperatures are shown in red and lower-than-normal temperatures are shown in blue. Each frame represents global temperature anomalies (changes) averaged over the five years previous to that particular year. Credit: NASA Goddard Space Flight Center/NASA Scientific Visualization Studio/NASA Goddard Institute for Space Studies. “Global warming” refers to the long-term warming of the planet. Global temperature shows a well-documented rise since the early 20th century and most notably since the late 1970s. Worldwide, since 1880 the average surface temperature has gone up by about 0.8 °C (1.4 °F), relative to the mid-20th-century baseline (of 1951-1980). “Climate change” encompasses global warming, but refers to the broader range of changes that are happening to our planet. These include rising sea levels, shrinking mountain glaciers, accelerating ice melt in Greenland, Antarctica and the Arctic, and shifts in flower/plant blooming times. These are all consequences of the warming, which is caused mainly by people burning fossil fuels and putting out heat-trapping gases into the air. The terms “global warming” and “climate change” are sometimes used interchangeably, but strictly they refer to slightly different things.

No. The sun can influence the Earth’s climate, but it isn’t responsible for the warming trend we’ve seen over the past few decades. The sun is a giver of life; it helps keep the planet warm enough for us to survive. We know subtle changes in the Earth’s orbit around the sun are responsible for the comings and goings of the ice ages. But the warming we’ve seen over the last few decades is too rapid to be linked to changes in Earth’s orbit, and too large to be caused by solar activity. In fact, recently (2005-2010) the sun has become less active, while temperatures have marched upwards. One of the “smoking guns” that tells us the sun is not causing global warming comes from looking at the amount of the sun’s energy that hits the top of the atmosphere. Since 1978, scientists have been tracking this using sensors on satellites and what they tell us is that there has been no upward trend in the amount of the sun’s energy reaching Earth. Global surface temperature (top, blue) and the sun's energy received at the top of Earth's atmosphere (red, bottom), from 1978 to 2009. The amount of solar energy received at the top of our atmosphere has followed its natural 11-year cycle of small ups and downs, but with no net increase. Over the same period, global temperature has risen markedly. It is therefore extremely unlikely that the sun has been behind the global temperature trend we’ve seen over several decades. Credit: NOAA National Climatic Data Center. A second smoking gun is that if the sun were responsible for global warming, we would expect to see warming throughout all layers of the atmosphere, from the surface all the way up to the upper atmosphere (stratosphere). But what we actually see is warming at the surface and cooling in the stratosphere. This is consistent with the warming being caused by a build-up of heat-trapping gases near the surface of the Earth, and not by the sun getting “hotter.”

Temperature data from four international science institutions. All show rapid warming in the past few decades. According to NASA data, 2013 tied with 2009 and 2006 for the seventh warmest year since 1880, continuing a long-term trend of rising global temperatures. With the exception of 1998, the 10 warmest years in the 134-year record all have occurred since 2000, with 2010 and 2005 ranking as the warmest years on record. Credit: NASA. Three of the world’s most complete temperature tracking records – from NASA’s Goddard Institute for Space Studies, the National Oceanic and Atmospheric Administration’s National Climactic Data Center and the UK Meteorological Office’s Hadley Centre – begin in 1880. Prior to 1880, temperature measurements were made with instruments like thermometers. The oldest continuous temperature record is the Central England Temperature Data Series, which began in 1659, and the Hadley Centre has some measurements beginning in 1850, but there are too few data before 1880 for scientists to estimate average temperatures for the entire planet. Data from earlier years is reconstructed from proxy records like tree rings, pollen counts and ice cores. Because these are different kinds of data, scientists generally don’t put proxy-based estimates on the same charts as the “instrumental record.” The above-mentioned agencies and others collect temperature data from thousands of weather stations worldwide, including over the ocean, in Antarctica and from satellites. However, instruments are not perfectly distributed around the globe, and some measurement sites have been deforested or urbanized since 1880, affecting temperatures nearby. Each agency uses algorithms to filter the effects of these changes out of the temperature record and interpolate where data is sparse, like over the vast southern ocean, when calculating global averages. Generally, all three datasets agree quite closely (see graph above) and are in agreement on the trend of global warming since the Industrial Revolution.

Some people say “weather is what you get” and “climate is what you expect.” “Weather” refers to the more local ch anges in the climate we see around us, on short timescales from minutes to hours to days to weeks. Examples are familiar – rain, snow, clouds, winds, thunderstorms, heat waves and floods. “Climate” refers to longer-term averages (they may be regional or global), and can be thought of as the weather averaged over several seasons, years or decades. Climate change is harder for us to get a sense of because the timescales involved are much longer, and the impact of climate changes can be less immediate. Examples of climate change include several drier-than-normal summers, a trend of, say, winters becoming milder from our grandparents’ childhood to our own, or variations in effects like El Niño or La Niña. Credit: Creative Travel Projects/Shutterstock.com

Yes, the vast majority of actively publishing climate scientists – 97 percent – agree that humans are causing global warming and climate change. Most of the leading science organizations around the world have issued public statements expressing this, including international and U.S. science academies, the United Nations Intergovernmental Panel on Climate Change and a whole host of reputable scientific bodies around the world. A list of these organizations is provided here https://climate.nasa.gov/scientific-consensus/