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What is Climate Change?

Earth’s climate has changed throughout its history, but the science leaves no doubt: human activities, not natural phenomena, are the driving force behind the exceptionally rapid warming that we are witnessing today.

Greenhouse gases (GHG) in Earth’s atmosphere act like a blanket, maintaining the planet’s temperature within a range where life can flourish. Since the Industrial Revolution, however, we have added excessive amounts of GHGs, thickening this blanket and causing Earth’s surface temperature to rise.

Figure A.1 Excess GHGs in Earth’s atmosphere trap more heat from the sun

Human activities are steadily increasing the atmospheric concentration of GHGs, particularly carbon dioxide, methane, nitrous oxides, and fluorinated gases. Energy production and combustion, primarily fossil fuels used in vehicles, buildings and industrial facilities, contributes 73% of these excess emissions. Land-clearing, forestry, and agriculture add another 18%. Atmospheric carbon dioxide is now at its highest level in at least 800,000 years.

Over the past 12,000 years, Earth’s temperature has fluctuated by only about half a degree Celsius, providing a stable environment for the development of complex societies and civilizations. However, we are now leaving this stable zone, as the average temperature rises at an unprecedented rate.

Figure A.2 Atmospheric carbon dioxide (CO2) levels over the past 800,000 years

Human activities have already caused the planet’s average temperature to increase by about 1.1°C.This warming is melting ice sheets, increasing sea levels, and causing more frequent and extreme weather events both locally and around the world. Climate change is already causing widespread damage to ecosystems and threatening one quarter of Earth’s plant and animal species with extinction.

Without immediate and comprehensive action across all levels of government, the Earth’s temperature is projected to rise by about 2.8°C by 2050. If governments roll back current policies, temperatures could soar by 4°C or more — a level unseen on Earth for millions of years, long before modern humans existed.

However, if governments pursue aggressive action now, warming could be limited to between 1.5°C and 2°C, a range that would result in far less damage. Every fraction of a degree of warming avoided by reducing GHG emissions will reduce the harm suffered by people and other species. Furthermore, investing in emissions reduction and adaptation to climate change impacts will save money for governments and households, create economic opportunities, and improve environmental and human health.