Factors that impact Earth's climate

• Greenhouse gases in the atmosphere.
• Solar flux, i.e. changes in solar energy over time;
• Changes in Earth's orbital cycle
• Aerosols from volcanic and human activity.
• Changes in land usage. This can impact the earth's albedo (reflectivity).

Measuring the health of Earth's climate

In relation to climate change, average temperature is the key metric used to assess Earth's climate health. On top of this, the Global Climate Observing System (GCOS) has identified 54 Essential Climate Variables (ECVs) that scientists use to better understand Earth’s climate. ECVs are "a physical, chemical or biological variable or a group of linked variables that critically contributes to the characterization of Earth’s climate".

Climate models are also helpful in predicting the future direction of the climate.

Measuring temperature

• Temperature levels are measured at ground sites, at sea (ships and buoys), and sometimes by satellites.
• Temperature data sets are complex and have different strengths and weaknesses. e.g. Global surface temperature data: HadCRUT4 and CRUTEM4 | Climate Data Guide

Measuring carbon dioxide

• Carbon dioxide levels are measured by both ground-sites and satellites. Historical levels are measured by like at air bubbles trapped in ice cores (to up to 800,000 years back).
• We started measuring carbon dioxide levels in the atmosphere from the top of an extinct volcano in Hawaii in 1958. This resulted in the Keeling Curve. We now measure CO₂ levels in the atmosphere at around 100 other sites around the world.
• Researchers have even reconstructed CO₂ levels from 66 million years ago by examining the chemical makeup of fossils of tiny marine organisms to understand ancient warming.