7.6 Evidence for Climate Change

Notes

There is overwhelming scientific consensus, supported by major scientific organizations worldwide, that Earth is warming and that human activities, mainly burning fossil fuels, are the dominant cause.

• Temperature: global average surface temperature has risen about 1.1 to 1.2 $^\circ$C since the late 1800s. The last decade was the warmest on record.
• Ice and sea level: glaciers, Arctic sea ice, and the Greenland and Antarctic ice sheets are losing mass. Sea level is rising because of melting land ice and thermal expansion of warming water.
• Ocean acidification: the ocean absorbs about a quarter of emitted $CO_2$, lowering its pH and harming corals and shellfish.
• Ice cores trapped air bubbles show $CO_2$ and temperature levels going back about 800,000 years. Today's $CO_2$ is higher than at any time in that record.
• Isotope evidence shows the extra atmospheric carbon comes from fossil fuels.
• Impacts: more heat waves and intense rainfall, droughts, wildfires, coral bleaching, shifting species ranges.
• Responses: mitigation (reducing emissions) and adaptation (adjusting to changes). The Paris Agreement (2015) set a goal to limit warming to well below 2 $^\circ$C. People debate which policies are best, such as carbon taxes, cap-and-trade, regulations, subsidies for clean energy, and how quickly to act and who should pay.
• Example: tide gauges and satellites both show sea level rising.