Building a Climate Model

	Next, to understand how sunlight, air, water, and land come together to create Earth’s climate, scientists build climate models—computer simulations of the climate system. Climate models include the fundamental laws of physics—conservation of energy, mass, and momentum—as well as dozens of factors that influence Earth’s climate. Though the models are complicated, rigorous tests with real-world data hone them into robust tools that allow scientists to experiment with the climate in a way not otherwise possible. For example, when scientists at NASA’s Goddard Institute for Space Studies (GISS), NASA’s division spearheading climate modeling efforts, put measurements of volcanic particles from Mount Pinatubo’s 1991 eruption into their climate models well after the event, the models reported that Earth would have cooled by around 0.5°C a year or so later. The prediction matched cooling that had been observed around the globe after the eruption.
	As the models reconstruct events that match the climate record, researchers gain confidence that the models are accurately duplicating the complex interactions that drive Earth’s climate. Scientists then experiment with the models to gain insight into what is driving climate change. By experimenting with the models—removing greenhouse gases emitted by the burning of fossil fuels or changing the intensity of the Sun to see how each influences the climate— scientists can use the models to explain Earth’s current climate and predict its future climate. So far, the only way scientists can get the models to match the rise in temperature seen over the past century is to include the greenhouse gases that humans have put into the atmosphere. This means that, according to the models, humans are responsible for most of the warming observed during the second half of the twentieth century.		Mount Pinatubo’s 1991 eruption pumped volcanic gases high into the atmosphere. The gases interacted with water vapor to form a reflective shade of aerosol particles (top graph) that stretched far beyond the Philippines, where the volcano is located. The global average temperature dipped half a degree Celsius until the particles (sulfates) cleared a few years later. Scientists test and refine global climate models by comparing model predictions of temperature change after events like the eruption to actual observations (bottom graph). When models reliably match observations, scientists gain confidence that the models accurately represent Earth’s climate system. (NASA graphs by Robert Simmon, based on data from NASA Goddard Institute for Space Studies.)

Major Climate Influences	Effect on Surface Temperature	Explanation	Source	Related EO Articles
Anthropogenic Greenhouse gases	Warming	Gases absorb energy emitted by the Earth, re-radiating heat toward the surface.	Burning fossil fuels in factories and vehicles, fertilizers, decomposing trash, livestock	Carbon Cycle
Ozone	Warming	Ozone absorbs sunlight, so a depletion of ozone cools the atmosphere, while a build-up of ozone warms the surface.	Chlorofluorocarbons destroy stratospheric ozone, while the interaction of sunlight with certain pollutants creates surface ozone	Tango in the Atmosphere; Ozone
Surface Albedo	Cooling	Albedo is the percent of sunlight the surface reflects. Deforestation increases albedo. Soot that falls on snow and ice reduces albedo. The effect of forest loss outweighs the effect of the soot, resulting in net cooling.	Land use and soot on snow	Arctic Reflection; Earth’s Albedo in Decline
Aerosols (particle pollution)	Cooling	Particles in the atmosphere shade the Earth’s surface, offsetting global warming as much as 40 percent. Soot absorbs heat and warms the atmosphere.	Volcanoes, burning fossil fuel, other human sources, fires, dust	Aerosols & Climate Change
Aerosols (Cloud albedo effect)	Cooling	Because human-produced aerosols are smaller and more numerous than natural aerosols, they increase the amount and brightness of clouds.	Clouds formed around pollution released by cars, factories, power plants, etc.	Clouds are Cooler than Smoke; Changing Our Weather One Smokestack at a Time
Solar Variability	Warming and Cooling	An increase in sunlight brings more energy to the Earth and vice-versa.	Natural solar cycles	ACRIMSAT; Solar Max; SORCE; Under a Variable Sun

	But why do scientists trust results from climate models when models seem to have so much trouble forecasting the weather? It turns out that trends are easier to predict than specific events. Weather is a short-term, small-scale set of measurements of environmental conditions, while climate is the average of those conditions over a large area for a long time. The difference between predicting weather and climate is similar to the difference between predicting when a particular person will die versus calculating the average life span of an entire population. Given the large number of variables that influence conditions in Earth’s lower atmosphere, and given that chaos also plays a larger role on shorter and smaller scales of time and space, weather is much harder to predict than the averages that make up climate. However, the longer the time scale, the harder it becomes to predict climate. Scientists understand how certain processes that drive Earth’s climate work now, and so they can accurately predict how events like Pinatubo’s eruption will cool the globe’s average temperature. But they don’t understand how every aspect of the climate system will change as the planet warms. Feedback loops—in which change in one part of the climate system produces change in another part—make climate harder to forecast as scientists look farther into the future. For example, what will happen to clouds as Earth warms? Will high-flying, heat-absorbing clouds that would cause additional heating become more frequent than dense, sunlight-blocking clouds? Will changes be regional or global, and how will they affect global climate? As of now, scientists can’t answer these questions, and the uncertainties mean that global climate models provide a range of predictions instead of a highly detailed forecast.		Models integrate the many factors that influence Earth’s climate to determine how they work together to create today’s climate and how they might influence climate change in the future.
	Observing Global Warming Climate models and paleoclimate information tell scientists what kinds of symptoms to look for when diagnosing global warming. Ocean temperatures and acidity should rise as the oceans soak up more heat and carbon dioxide. Global temperatures are predicted to increase, with the largest temperature increases over land and at the poles. Glaciers and sea ice will melt and sea levels will rise. Like a patient in a hospital, Earth is closely monitored for these symptoms by a fleet of satellites and surface instruments. NASA satellites record a host of vital signs including atmospheric aerosols (particles from things like factories, fires, or erupting volcanoes), atmospheric gases, energy from Earth’s surface and the Sun, ocean surface temperatures, global sea levels, the extent of ice sheets, glaciers and sea ice, plant growth, rainfall, cloud structure, and more. On the ground, networks of weather stations maintain temperature and rainfall records, and buoys measure deep ocean temperatures.		Climate is what you expect; weather is what you get. This graph compares long-term average high and low temperatures (dark lines) to the actual daily high and low temperatures in New York City’s Central Park during 2006. Although the average temperatures vary gradually as the seasons change, temperatures fluctuate wildly from day-to-day. (NASA graph by Robert Simmon, based on data from the National Weather Service Forecast Office.)
	Along with paleoclimate data, these sources reveal that the planet has been warming for at least the last 400 years, and possibly the last 1000 years. As of now, warming after 1950 cannot be explained without accounting for greenhouse gases; natural influences such as volcanic eruptions or changes in the Sun’s output cannot account for the observed temperatures changes. Occasional violent volcanic eruptions, such as Mt. Pinatubo, pump gases like sulfur dioxide and aerosols high into the atmosphere where they can linger for more than a year, reflecting sunlight and shading Earth’s surface. The cooling influence of this aerosol “shade” is greater than the warming influence of the volcanoes’ greenhouse gas emissions, and therefore such eruptions cannot account for the recent warming trend. An increase in solar output also falls short of explaining recent warming. NASA satellites have been measuring the Sun’s output since 1978, and while the Sun’s activity has varied a little, the observed changes were not large enough to account for the warming recorded during the same period. Climate simulations of global temperature changes based only on solar variability and volcanic aerosols since 1750—omitting greenhouse gases— are able to fit the record of global temperatures only up until about 1950.		About half the carbon dioxide emitted into the air from burning fossil fuels dissolves in the ocean. While this process reduces the amount of greenhouse gases in the atmosphere, it raises the acidity of ocean water (just like carbonated water, which is acidic). This map shows the total amount of human-made carbon dioxide in ocean water from the surface to the sea floor. Blue areas have low amounts, while yellow regions are rich in anthropogenic carbon dioxide. High amounts occur where currents carry the carbon-dioxide-rich surface water into the ocean depths. (Map adapted from Sabine et al., 2004.)
	The only viable explanation for warming after 1950 is an increase in greenhouse gases. It is well established theoretically why carbon dioxide, methane, and other greenhouse gases should heat the planet, and observations show that they have. Predicting Future Warming Evidence for Global Warming		Satellite measurements of the Sun’s activity since 1978 reveal the Sun’s eleven-year sunspot cycle. When the Sun is more active, it sends slightly more energy to the Earth, and energy levels dip when the Sun is quieter. Though the Sun’s activity has varied over the past three decades, the variation is too small to explain the rapid warming seen on Earth during the same period. (Graph adapted from the PMOD World Radiation Center.)