The response of the tropics to increasing CO2 is a central topic in climate change research, as tropical climate onditions can have far-reaching effects and they set the background for changes in the character variations, such as those of El Niño. Theoretical and modeling understanding, and observational evidence for long-term changes to the tropical climate system will be highlighted. Changes in the oceanic circulation and thermal structure will be iscussed, as will changes in atmospheric circulation and precipitation patterns - including changes in the Hadley and Walker Circulations. The change to the tropical Pacific mean state to greenhouse warming is complex, controlled by both oceanic and atmospheric processes. In models with a simplified representation of atmospheric physics, feedbacks originating in the ocean drive the system to a “La Niña-like” state. In models with atmospheric general circulation components, thermodynamic constraints result in a reduction of the strength of the atmospheric overturning circulation – manifest primarily in the zonally-asymmetric (Walker) rather than zonal-mean (Hadley) component. In these models changes over the tropical Pacific Ocean resemble “El Niño-like” conditions, although the mechanisms are fundamentally different from those of El Niño, as are many of the impacts. Even though modeling studies can help reconcile aspects of the diverging theoretical understanding, a true reconciliation requires observational evidence. Observations of sea level pressure indicate that over the 20th Century the Pacific Walker circulation has weakened; however, differing reconstructions of historical SST are inadequate to distinguish between an increase or decrease in East-West SST gradient across the Pacific. We outline what we view as a way forward, with targeted reconstructions and paleo-proxy observations, to reconcile these diverging views. Implications of the modeled changes are also discussed.

Close to twenty percent of global carbon emissions can be traced to deforestation in the tropics. By preserving forests(BS1)(BS2), reforesting marginal lands(BS3), and improving agricultural practices, carbon sequestration can be enhanced and carbon emissions reduced. Hence the United Nations Framework Convention on Climate Change (UNFCCC) has taken on the issues of Reducing Emissions from Deforestation and Forest Degradation (REDD) in developing countries and incorporating changes in carbon storage and emissions from Agriculture, Forestry, and Other Land Use (AFOLU) in intergovernmental agreements. Previous negotiations on the inclusion of forests for climate mitigation in developing countries have been held back for many reasons including technical challenges of monitoring carbon stocks with high certainty, and issues of leakage, additionality, and permanence (BS4). Moreover, despite much effort to date by national and international organizations, the requisite national and global information to underpin such agreements and financial markets is lacking. Some useful lessons can be learned from a onsideration of the information delivery of weather forecasts and services to the public as a model for forest carbon accounting systems. The responsibility and roles of all the institutions involved - from national governments and various international organizations and agreements to foundations, non-governmental organizations, universities, and the private sector, are similar. The comparison of how weather forecasts and other services are developed and delivered to users may help in defining the issues to be addressed in forest carbon accounting. In this talk I will outline what is needed to establish national forest carbon accounting systems that will provide estimates of forest carbon storage and emissions information with a high degree of certainty