Evidence

History

Climate science was pioneered by many of the same scientists that laid the foundation for modern chemistry and physics. Human-driven climate change was predicted in the mid-1800s, but hard evidence of human-induced warming was not discovered until the late 20th century.

After World War II, climate science received a jolt from an unlikely source: the U.S. military. The military became a major funder of scientific research because Cold War weapons depended on an intimate knowledge of earth science, such as nuclear weapons and heat seeking missiles. The weapons research inadvertently contributed to the collection of climate data. With unprecedented precision, scientists could measure gases and heat in the air. Over time, the data records helped connect pollution to increasing temperatures. Despite popular media reports of global cooling in the 1970s, it was never a mainstream view within the scientific community.

As scientists became more convinced of human-driven climate change, the United Nations formed the Intergovernmental Panel on Climate Change (IPCC) to inform governments about the science in 1988. This body is tasked with reviewing tens of thousands of scientific studies and synthesizing the findings into reports. Hundreds of the world’s top scientists — include those from MIT, Stanford and even ExxonMobil — work on publishing these reports every few years. To this day, they continue to release periodic updates on developments in climate science. The 2014 IPCC report was deemed the most comprehensive review of any scientific subject in human history. According to the report, the overwhelming scientific evidence shows that “human influence on the climate system is clear” and “has been the dominant cause of the observed warming since the mid-20th century.”

Evidence

The IPCC’s results are supported by observable changes today: more intense heat waves, shorter winters, and larger wildfires. These events are a result of complex interactions and feedbacks between greenhouse gases in the atmosphere, ocean, land, ice, and living things. NASA and MIT software models, based on complex mathematical formulas run on supercomputers, predict more warming and extreme weather if climate change pollution remains unchecked. In recent years, professionals from all sectors are seeing the impacts of climate change.

Medical professionals have observed public health consequences of climate change; the U.S. military has emerged as a vocal advocate for climate adaptation; and economic reports such as a PriceWaterhouseCoopers report have outlined the serious economic consequences associated with climate change.

Reconstructing the past

Drawing from numerous sources of scientific and historical evidence, research efforts around the world have recreated past records of temperature and greenhouse gases. Scientists have determined that global carbon dioxide concentrations have approached record high levels that people have never experienced. They combine satellite data and other instrumental observations with ‘natural’ methods that approximate past climatic conditions, such as from ice cores, marine sediment, and tree rings. For example, scientists can link the carbon building up in our atmosphere back to fuels burned in our power plants and cars using subatomic “fingerprints”.

Piecing together clues like these, scientists identify human activity as the main contributor to recent climate change. Natural factors on the Earth’s climate, such as changes in the earth’s orbit and sun, cannot explain the recent rise in temperatures. Only the sudden increase in global carbon dioxide by people can plausibly explain the levels of observed warming.