- There are several elements in the climate system that could pass a tipping point this century due to human activities, leading to abrupt and/or irreversible change.
- 1°C global warming (above 1980-1999) carries moderately significant risks of passing large-scale tipping points, and 3°C global warming would give substantial or severe risks. …
- The reconstruction of past climate reveals that the recent warming observed in the Arctic, and in the Northern Hemisphere in general, are anomalous in the context of natural climate variability over the last 2000 years.
- New ice-core records confirm the importance of greenhouse gases for past temperatures on Earth, and show that CO2 levels are higher now than they have ever been during the last 800,000 years. …
Blue line: estimates of Arctic air temperatures over the last 2,000 years based on proxy records from lake sediments, ice cores and tree rings.
[Green line:] best fit long-term cooling trend for the period ending 1900.
[Red line:] recent warming based on actual observations.
(Science, modified by the University Corporation for Atmospheric Research)
- Global mean air-temperature is projected to warm 2-7°C above pre-industrial by 2100. …
- There is a very high probability of the warming exceeding 2°C unless global emissions peak and start to decline rapidly by 2020.
- Warming rates will accelerate if positive carbon feedbacks significantly diminish the efficiency of the land and ocean to absorb our CO2 emissions.
- Many indicators are currently tracking near or above the worst case projections from the IPCC AR4 set of model simulations. …