The Southern Annular Mode (SAM) is the primary pattern of climate variability in the Southern Hemisphere 1,2 , influencing latitudinal rainfall distribution and temperatures from the subtropics to Antarctica. The positive summer trend in the SAM over recent decades is widely attributed to stratospheric ozone depletion 2 ; however, the brevity of observational records from Antarctica 1 -one of the core zones that defines SAM variability-limits our understanding of long-term SAM behaviour. Here we reconstruct annual mean changes in the SAM since AD 1000 using, for the first time, proxy records that encompass the full mid-latitude to polar domain across the Drake Passage sector. We find that the SAM has undergone a progressive shift towards its positive phase since the fifteenth century, causing cooling of the main Antarctic continent at the same time that the Antarctic Peninsula has warmed. The positive trend in the SAM since ∼AD 1940 is reproduced by multimodel climate simulations forced with rising greenhouse gas levels and later ozone depletion, and the long-term average SAM index is now at its highest level for at least the past 1,000 years. Reconstructed SAM trends before the twentieth century are more prominent than those in radiative-forcing climate experiments and may be associated with a teleconnected response to tropical Pacific climate. Our findings imply that predictions of further greenhouse-driven increases in the SAM over the coming century 3 also need to account for the possibility of opposing e ects from tropical Pacific climate changes.

Carbon dioxide (CO2) is a kinetically and thermodynamically stable molecule. It is easily formed by the oxidation of organic molecules, during combustion or respiration, but is difficult to reduce. The production of reduced carbon compounds from CO 2 is an attractive proposition, because carbon-neutral energy sources could be used to generate fuel resources and sequester CO 2 from the atmosphere. However, available methods for the electrochemical reduction of CO2 require excessive overpotentials (are energetically wasteful) and produce mixtures of products. Here, we show that a tungsten-containing formate dehydrogenase enzyme (FDH1) adsorbed to an electrode surface catalyzes the efficient electrochemical reduction of CO 2 to formate. Electrocatalysis by FDH1 is thermodynamically reversible-only small overpotentials are required, and the point of zero net catalytic current defines the reduction potential. It occurs under thoroughly mild conditions, and formate is the only product. Both as a homogeneous catalyst and on the electrode, FDH1 catalyzes CO 2 reduction with a rate more than two orders of magnitude faster than that of any known catalyst for the same reaction. Formate oxidation is more than five times faster than CO 2 reduction. Thermodynamically, formate and hydrogen are oxidized at similar potentials, so formate is a viable energy source in its own right as well as an industrially important feedstock and a stable intermediate in the conversion of CO2 to methanol and methane. FDH1 demonstrates the feasibility of interconverting CO2 and formate electrochemically, and it is a template for the development of robust synthetic catalysts suitable for practical applications.electrocatalysis ͉ formate dehydrogenase ͉ formate oxidation ͉ protein film voltammetry ͉ carbon dioxide reduction C arbon dioxide (CO 2 ) is a kinetically and thermodynamically stable molecule that is difficult to chemically activate. As the unreactive product of the combustion of carbon-containing molecules, such as fossil fuels, and biological respiration, it is accumulating in the atmosphere and is a major cause of concern in climate-change scenarios (1, 2). Consequently, catalysts that are able to sequester CO 2 from the atmosphere rapidly and efficiently, to generate reduced carbon compounds for use as fuels or chemical feedstocks, have long been sought after. Organometallic complexes that insert CO 2 into an M-H bond and the use of supercritical CO 2 to facilitate its hydrogenation are two promising approaches to the development of a homogeneous catalytic process (3-5). Extensive efforts to develop electrode materials for the direct, electrochemical reduction of CO 2 have been made also, but so far, they require the application of extreme potentials (are inefficient energetically) or they are nonspecific and produce mixtures of products (4, 6, 7). An efficient and specific method for reducing CO 2 electrochemically has obvious applications in a future energy economy based on the cyclic reduction and oxidation of simple carbon compound...