Distinguishing Natural Climate Variability from Anthropogenic Climate Change

Sediment cores collected from the Atlantic Ocean contain
microfossils and geochemical proxies that allow a
reconstruction of detailed sea-surface temperature records
for the last 20,000 years. These reconstructions provide
information for key climate variables (e.g. rates of
temperature change) in the past that help us to
understand natural climate variability and to project what
conditions might be like in the future.

An important question in climate change research is whether we can distinguish the human fingerprint on climate from natural climate variability. Solar activity, volcanic emissions and greenhouse gases, including those from human activities, all affect the radiation and energy balance of the Earth. Variations in the energy balance lead to changes in the distribution and patterns of air temperature, rainfall, hydrology, polar sea ice and glacier mass. Internal modes of climate variability, such as ENSO and the North Atlantic Oscillation (NAO) can cause large year-to-year and decade-to-decade changes in temperature and rainfall.

Distinguishing human-induced climate change associated with carbon emissions and land use change from natural climate variability requires integrated research efforts that rely on climate modeling and paleoclimate reconstructions based on data from analyses of tree rings, ice cores, marine and terrestrial sediments, glaciers and instrumental records. In essence, this research aims to sort out the contributions from natural radiative forcing and internal climate processes from those caused either directly or indirectly by human activity. There is general and widely held scientific consensus that the observed trends in atmospheric and ocean temperature, sea ice, glaciers and climate extremes during the last century cannot be explained solely by natural climate processes and so reflect human influences.