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Abstract

The energy radiated by the Earth toward space does not compensate the incoming radiation from the Sun leading to a small positive energy imbalance at the top of the atmosphere (0.4–1 Wm–2). This imbalance is coined Earth’s Energy Imbalance (EEI). It is mostly caused by anthropogenic greenhouse gas emissions and is driving the current warming of the planet. Combined with surface temperature measurements the EEI measurement informs on the sensitivity of the climate system to GHG emissions (the so-called climate sensitivity). Thus monitoring precisely the EEI is critical to assess the current status of climate change, estimate the climate sensitivity and by this mean evaluate the future evolution of climate. But the monitoring of EEI is challenging as it is two orders of magnitude smaller than the radiation fluxes in and out of the Earth system. Over 90% of the excess energy that is gained by the Earth in response to the positive EEI accumulates into the ocean in the form of heat such that the monitoring of Ocean Heat Content (OHC) and its long-term change provides a precise estimate of EEI. Today, global OHC changes can be tracked from space with a combination of the altimetric measurement of sea level change and the gravimetric measurement of ocean mass change. In this talk we review this current space method to estimate global OHC changes and evaluate its relevance to derive EEI estimates on different time scales. We compare its performance with an independent estimate from direct observations of in situ temperature. Then, we use both the space based and in situ based estimate of EEI along with the surface temperature record to derive estimates of the 20^th century mean effective climate sensitivity. Accounting for the internal variability (with an explicit representation of the so called “pattern effect”) we derive from our observed 20^th century effective climate sensitivity an observational constraint on the climate sensitivity of 5.4 [2.4;35.6] K (median, 5-95% CI) with the in situ data and of 3.4 [1.5;20.8]° with the space data. In order to explore these discrepancies, we also calculate time variations of the climate feedback parameter, to which depends the climate sensitivity, with a sea level reconstruction since 1900, by deriving the EEI directly from the thermosteric component of sea level rise, and from the difference between the sea surface height variations and the ocean mass budget contribution to sea level. First we show that constraints on this parameter requires observing systems owing a precise monitoring of sea level and its contributions in order to precisely close the sea level budget. Second we show that the variability of this parameter responds to external forcings such as major volcanic eruptions, but also to the internal climate variability. For the time being, space geodesy observations are still too short to observe time-variations of the climate feedback parameter, but results are promising enough to expect to do it in the next years.