evidence: Scientists from around the world serve as part of the Intergovernmental Panel on Climate Change (IPCC). These scientists have found that from 1900-2020, the world's surface air temperature increased an average of 1.1° Celsius (nearly 2°F) due to burning fossil fuels that releases carbon dioxide and other greenhouse gases into the atmosphere. This may not sound like very much change, but this warming is unprecedented in over 2000 years of records. Even one degree can impact the planet in many ways. Climate models predict that Earth's global average temperature will rise an additional 4° C (7.2° F) during the 21st Century if greenhouse gas levels continue to rise at present levels. Without swift action to reduce greenhouse gas emissions, models project that holding global average temperatures to within a 1.5-2.0°C (2.7-3.6°F) increase may no longer be possible.