Environment Counts | Climate Trends

Recent observations show that greenhouse gas emissions and many aspects of the climate are changing near the upper boundary of the IPCC range of projections. Many key climate indicators are already moving beyond the patterns of natural variability within which contemporary society and economy have developed and thrived. These indicators include global mean surface temperature, sea-level rise, global ocean temperature, Arctic sea ice extent, ocean acidification, and extreme climatic events. With unabated emissions, many trends in climate will likely accelerate, leading to an increasing risk of abrupt or irreversible climatic shifts. sourceClimate Change, Synthesis Report, University of Copenhagen]

The Intergovernmental Panel on Climate Change (IPCC) concluded in 20072 that climate change is, without doubt, occurring and that the Earth is warming. More importantly, the IPCC concluded that there is over 90% probability that this global warming is primarily caused by human activities – the most important of these being the emission of greenhouse gases and the clearing of natural vegetation. Since 2007, reports comparing the IPCCprojections of 1990 with observations show that some climate indicators are changing near the upper end of the range indicated by the projections or, as in the case of sea level rise (Figure 1), at even greater rates than indicated by IPCC projections. Grasping the significance of such observations requires an understanding of climate change that goes beyond the warming of the atmosphere.

The climate is largely controlled by the flows of heat entering and leaving the planet and the storage of heat in the various compartments of the Earth System – ocean, land, atmosphere, snow/ice. This heat ultimately comes from the sun. Only a very small amount of the heat is stored in the atmosphere (Figure 2); by far the largest amount of heat stored at the Earth’s surface is found in the ocean. The heat flux into the ocean proceeds more slowly than into the atmosphere. However, given that the ocean stores so much heat, a change in ocean temperature, which reflects a change in the amount of heat stored in the ocean, is a better indicator of change in the climate than changes in air temperature.

Figure 3 shows the trend in surface air temperature in recent decades. 2008 was comparatively cooler than the immediately preceding years, primarily because there was a minimum in the cycle of the sun’s magnetic activity (sun spot cycle) and a La Niña event in 2007/2008. Nevertheless, the long-term trend of increasing temperature is clear and the trajectory of atmospheric temperature at the Earth’s surface is proceeding within the range of IPCC projections.

Since the last IPCC report, updated trends in surface ocean temperature and heat content have been published. These revised estimates show (Figure 4) that the ocean has warmed significantly in recent years. Current estimates indicate that ocean warming is about 50% greater than had been previously reported by the IPCC. The new estimates help to better explain the trend in sea level that has been observed in recent decades as most of the sea level-rise observed until recently has been the result of thermal expansion of sea water.
Climate Change, Synthesis Report, University of Copenhagen