Environmental Defense Fund

How we know human activity is causing warming

We know the planet is warming — scientists have a clear understanding why

The theory of global warming is nothing new. The Nobel Prize-winning chemist Svante Arrhenius first proposed the idea of global warming in 1896. Carbon dioxide, he knew, traps heat in the Earth's atmosphere. He also knew that burning coal and oil releases carbon dioxide (CO₂).

Arrhenius speculated that continued burning of coal and oil would increase concentrations of CO₂ in the Earth's atmosphere, making the planet warmer. It's called the greenhouse effect.

What warms the Earth?

To determine what is causing today's rapid global warming, scientists have examined all the factors that can affect the Earth's temperature. There are essentially three factors that could be responsible for recent rapid global warming:

1. The sun
2. Earth's reflectivity
3. Greenhouse gases

Which of these is causing our current global warming?

It's not the sun: cause of little warming since 1750, none since 1980s

Ultimately, the climate system is powered by the sun: all else being equal, if you turn up the sun, you'll warm up the Earth. According to IPCC estimates, the sun has accounted for just a small portion of warming since 1750. A study of more recent solar activity has demonstrated that since about 1985 the sun has changed in ways that, if anything, should have cooled the planet—even as global temperatures have been rising. So the sun is not causing global warming.

It's not reflectivity: changes point to cooling, not warming

Around 30% of the sun's energy that reaches the Earth is reflected back into space. Changes in how much sunlight is absorbed, and how much is reflected, can affect global temperatures. Using satellite and land-based observations and computer models, scientists have calculated how Earth's reflectivity has changed over time.

These calculations suggest that human-produced particulate pollution, especially reflective sulfur-containing particles, have had a cooling effect on the climate, masking some of the warming effect of greenhouse gases. In fact, the slight decrease in global temperature between 1945 and 1975 was likely caused by a combination of rising particulate pollution and natural factors. Warming resumed after 1975 when industrialized countries began to clean up their particulate pollution while continuing to increase their greenhouse gas emissions.

As for human land use changes (primarily forest clearing for agriculture), they have on balance brightened the planet since 1750. This would have a cooling effect, yet we've seen warming. Changes in the frequency of volcanic eruptions, which can send reflective particles up into the stratosphere, also cannot explain the observed warming trend. So reflectivity is not causing global warming.

All the evidence points to greenhouse gases

That leaves the greenhouse effect as the only remaining scientific explanation for the rise in global temperatures in recent decades. We have direct measurements of CO₂ concentrations in the atmosphere going back more than 50 years, and indirect measurements (from ice cores) going back hundreds of thousands of years. These measurements confirm that concentrations are rising rapidly.

Historic CO₂ levels

The amount of carbon dioxide in the atmosphere is higher than at any time in measurable history, and predicted to increase dramatically this century. Source: GlobalChange.gov

Though natural amounts of CO₂ have varied from 180 to 300 parts per million (ppm), today's CO₂ levels are around 390 ppm. That's 30% more than the highest natural levels over the past 800,000 years. Increased CO₂ levels have contributed to periods of higher average temperatures throughout that long record. (Boden, Carbon Dioxide Information Analysis Center)

We also know the additional CO₂ in the atmosphere comes mainly from coal and oil, because the chemical composition of the CO₂ contains a unique "fingerprint."

As far as scientists are concerned, it's case closed: human activity is causing the Earth to get warmer, primarily through the burning of fossil fuels, with a smaller contribution from deforestation. All other scientific explanations for why the Earth is getting warmer have been eliminated.

World View of Human Impact on Temperature Since 1900

Continental and global temperatures modeled with and without human influence show the impact of human activity on global warming. View full-size. Source: IPCC 2007: WG1 AR4 Figure SPM.4

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