Climate Change 101

History of our changing climate

Yearly surface temperature compared to the 20th-century average from 1880–2022. Blue bars indicate cooler-than-average years; red bars show warmer-than-average years. NOAA graph, based on data from the National Centers for Environmental Information.

Since the early days of geology, scientists have understood that the earth’s climate is constantly changing. As early as the 1800s, scientists suggested that humans could in principle change the Earth's climate based on the "greenhouse" properties of some atmospheric gases. 

In the late 1960s, scientists discovered that carbon dioxide was increasing about 0.5% per year, and in the late 1970s early computer models were able to estimate the impact of continued carbon dioxide rise in temperature extremes, changing precipitation patterns, and sea level rise. 

By the 1990s compiled climate data confirmed that temperature increases in the 20th century were beyond what would be expected from natural variations. In the past decade, a large body of evidence has confirmed what is happening to the earth’s climate today and what it means for our future.

Research evidence: What we know for sure

There is unequivocal evidence the earth is warming at an unprecdented rate. While Earth’s climate has changed throughout its history, the current warming is happening at a rate not seen in the past 10,000 years.

97 percent of climate scientists agree that the earth is warming due to human activity. Every major scientific organization related to the climate – including the Intergovernmental Panel on Climate Change, the National Academy of Sciences, the American Meteorological Society, the American Geophysical Union, and the American Association for the Advancement of Science – have issued statements explaining that greenhouse gas emissions are the major contributor to climate change. 

What’s actually happening?

When sunlight reaches the earth’s surface, it can either be reflected back into space or absorbed by the earth, warming the planet. When sunlight is absorbed, it is called the greenhouse effect, a natural and important process that support the planet.

As humans across the globe burn increasing amounts of fossil fuel each year, we release larger amounts of greenhouse gases, including carbon dioxide, into the atmosphere. These gases amplify the greenhouse effect, leading to global warming. 

Scientists have documented that there is substantially more carbon dioxide in the atmosphere than before industrial revolution and that the Earth is currently hotter than ever before. Nine of the 10 warmest years since we began recording temperatures in 1880 have occurred since 2005, and the 5 warmest years on record have occurred since 2015.

As the planet warms, scientists have observed:

  • Significantly warmer air temperatures.
  • Melting of glaciers and sea ice.
  • Increases in ocean temperatures and higher sea levels.
  • More frequent and intense of extreme weather events.
  • Changes in our ecosystems, such as the length and timing of growing seasons and variations in migration patterns.

Complexity surrounding climate change

The earth’s climate is an extremely complex system. Researchers are still working to better understand how climate works and changes. 

Current data show there have been more extreme precipitation events and droughts in recent years, but the exact relationship between such events – particularly hurricanes – and climate warming remains unclear. Global-scale trends are easier to predict than local ones; the impacts of climate change on local precipitation is often poorly explained by existing models. 

The geologic record over thousands of years suggests that climate is potentially very sensitive; under certain conditions global climate may suddenly change quickly. Greenhouse gas concentrations are currently rising more quicky than ever before. Scientists don’t fully understand if there will be a tipping point when our climate responds more drastically to the substantial increase in greenhouse gases.

Effects of climate change

Over the next century, global climate change is expected to impact billions of people through:

  • Increased flooding as sea levels rise.
  • Increase exposure to extreme heat waves.
  • Poor air quality, which affects heart and lung health.
  • Increased prevalence of insects that spread disease, such as ticks and mosquitos.
  • More extensive droughts and storms.
  • Decreased food production.

Local impacts are more difficult to predict, but could include negative outcomes for agriculture, winter recreation, forestry, and fishing industries, and others. 

Though climate change can be a natural phenomenon, the rate of the current, human-induced climate change is comparatively fast and will likely affect humans and other organisms in ways that are difficult to adapt to.  

For more information

Climate Change Facts from the Cornell University College of Agriculture and Life Sciences

Videos on Climate Change by How Global Warming Works

Climate Change Indicators by the U.S. Environmental Protection Agency

Facts from the U.S. Global Climate Change Research Program