Climate Change 2013: The Physical Science Basis - Summary For Policymakers.pdf

Key findings include:

1. Warmth and Extreme Weather: Global temperatures have increased by about 0.9°C since pre-industrial times, with the most rapid warming occurring over the past 30 years. This trend is consistent with human activities, primarily greenhouse gas emissions.

2. Sea Level Rise: Global sea levels rose at an average rate of about 1.7 mm per year between 1901 and 2010, with accelerated rise since 1993. Projected future sea level rise ranges from 28 cm to 100 cm by 2100, depending on greenhouse gas emissions scenarios.

3. Atmospheric Composition: Concentrations of greenhouse gases, such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), have increased significantly since pre-industrial times. Human activities are the primary cause of these increases.

4. Ocean Warming and Acidification: The oceans have absorbed about 93% of the excess heat generated by greenhouse gas warming, leading to ocean warming and changes in ocean circulation. Increased absorption of CO2 is causing ocean acidification, which poses significant risks to marine ecosystems.

5. Glacial Retreat and Ice Sheet Loss: Glaciers worldwide are retreating, with some losing more than 50% of their mass since the mid-20th century. The Greenland and Antarctic ice sheets are contributing significantly to sea level rise.

6. Projected Future Climate Change: Under high emissions scenarios, global temperatures could increase by as much as 4.8°C by 2100. Extreme weather events, such as heatwaves, droughts, and heavy precipitation, are expected to become more frequent and intense.

These findings underscore the urgent need for action to mitigate greenhouse gas emissions and adapt to unavoidable climate change impacts. The report serves as a critical resource for policymakers in developing strategies to address this global challenge.