Less Than 2 °C Warming by 2100 Unlikely.

The recently published Intergovernmental Panel on Climate Change (IPCC) projections to 2100 give likely ranges of global temperature increase in four scenarios for population, economic growth and carbon use1. However these projections are not based on a fully statistical approach. Here we use a country-specific version of Kaya's identity to develop a statistically-based probabilistic forecast of CO2 emissions and temperature change to 2100. Using data for 1960-2010, including the UN's probabilistic population projections for all countries2-4, we develop a joint Bayesian hierarchical model for GDP per capita and carbon intensity. We find that the 90% interval for cumulative CO2 emissions includes the IPCC's two middle scenarios but not the extreme ones. The likely range of global temperature increase is 2.0-4.9°C, with median 3.2°C and a 5% (1%) chance that it will be less than 2°C (1.5°C). Population growth is not a major contributing factor. Our model is not a "business as usual" scenario, but rather is based on data which already show the effect of emission mitigation policies. Achieving the goal of less than 1.5°C warming will require carbon intensity to decline much faster than in the recent past

Less Than 2 °C Warming by 2100 Unlikely.

The recently published Intergovernmental Panel on Climate Change (IPCC) projections to 2100 give likely ranges of global temperature increase in four scenarios for population, economic growth and carbon use1. However these projections are not based on a fully statistical approach. Here we use a country-specific version of Kaya's identity to develop a statistically-based probabilistic forecast of CO2 emissions and temperature change to 2100. Using data for 1960-2010, including the UN's probabilistic population projections for all countries2-4, we develop a joint Bayesian hierarchical model for GDP per capita and carbon intensity. We find that the 90% interval for cumulative CO2 emissions includes the IPCC's two middle scenarios but not the extreme ones. The likely range of global temperature increase is 2.0-4.9°C, with median 3.2°C and a 5% (1%) chance that it will be less than 2°C (1.5°C). Population growth is not a major contributing factor. Our model is not a "business as usual" scenario, but rather is based on data which already show the effect of emission mitigation policies. Achieving the goal of less than 1.5°C warming will require carbon intensity to decline much faster than in the recent past

Tropical Precipitation and Cross-Equatorial Heat Transport in Response to Localized Heating: Basin and Hemisphere Dependence

A heat source is applied to different surface locations in a fully coupled climate model to study the cross-equatorial energy transport and tropical precipitation responses. Remarkably different tropical precipitation responses are seen, varying from a large shift toward, to a small shift away from, the heated hemisphere. These differences are dominated by changes in top-of-atmosphere radiation, with some contribution from changes in ocean cross-equatorial heat flux. The atmospheric fraction of the total cross-equatorial heat flux is consistently larger for Northern Hemisphere (NH) heating relative to Southern Hemisphere heating. This results in a larger tropical rainfall shift in response to NH heating. Positive shortwave radiative feedbacks, associated with a burn-off of low clouds in the North Pacific, also amplify the tropical rainfall response to NH heating. The Pacific Ocean dominates the ocean response to Southern Hemisphere heating, while the Atlantic Ocean dominates the ocean response to NH heating. Pain Language Summary When there is a change in the energy balance between the Northern and Southern Hemispheres, for example, because of aerosol emissions (or clean-up), then heat transport across the equator must shift to maintain energy balance. If that cross-equatorial heat transport is produced by changes in the atmospheric circulation (instead of ocean circulation), then tropical precipitation shifts toward the more heated hemisphere. We perturb a fully coupled atmosphere-ocean climate model with localized heating at the ocean surface in different hemispheres, latitudes, and ocean basins, and study the circulation and tropical precipitation responses. The largest response is from heating in the extratropical North Pacific; this is largely because of strong positive cloud feedbacks in this region and partially because changes in ocean heat transport contribute relatively little to the required change in cross-equatorial heat transport. In this climate model we find that tropical precipitation is more sensitive to Northern Hemisphere heating than Southern Hemisphere heating and more sensitive to heating in the North Pacific than the North Atlantic. This will have implications for the tropical precipitation response to aerosol clean-up, and perhaps to cloud feedbacks in different regions in response to global warming