Observations and Understanding of Recent Climate Change (Invited)

Observations of the atmosphere, land, ocean and ice from instrumental and paleoclimatic sources demonstrate that the Earth is warming. Models are able to simulate many elements of observed atmospheric, oceanic and cryospheric change over the past century, allowing us to distinguish human-induced from natural change. Ocean salinity measurements indicate that the contrast between fresh and salty regions over the oceans has increased over the past 60 years. Regions of high salinity where evaporation exceeds precipitation have become more saline, while salinity has decreased in regions where precipitation exceeds evaporation, which is consistent with the response of climate models to human-induced climate change. The summer minimum ice cover in the Arctic is declining rapidly. While simulations and observations show broadly consistent warming on multi-decadal timescales, the observed global-mean surface temperature has shown a much smaller warming trend over the past 15 years than over the past 30 to 60 years. The majority of CMIP5 model simulations show a temperature trend over 1998-2012 that is larger than the observed trend. This difference between simulated and observed trends could be caused by some combination of internal climate variability, missing or incorrect radiative forcing, and model response error. A challenge for the future will be to understand and quantify the causes of the decreased warming rate in the past 15 years and explore the implications for understanding climate variability and change on decadal and longer time scales.