It is important to quantify the long-term average state of the ocean. Eventually, the influence of anthropogenic climate change will be larger than that of ongoing natural variability. This transition is known as the emergence. But we are not there yet. The present oceanic signature of anthropogenic climate change is still comparable to, and thus difficult to disentangle from, natural and regional climate variability such as the El Niño Southern Oscillation, cycles in winds and sea surface temperatures over the tropi¬cal east Pacific Ocean.
Emergence will happen at different times in different places. For example, the tropics are already recording extreme temperatures, whereas the emergence is several decades away at mid-latitudes.
Natural climate variability can offset or amplify climate change trends temporarily (see ‘Reading the waves’). For example, an apparent5 slowing or ‘hiatus’ in global average temperature rise between 1998 and 2012 led some critics to downplay anthropogenic climate change. Natural variability also reflects more extreme conditions, such as latest strong El Niño warming event.
As anthropogenic climate change increases, periods of extreme conditions6 are expected to become more frequent, severe and lengthy. These will have adverse effects on marine ecosystems. For example, in 2011 the west coast of Australia encoun¬tered sea surface temperatures that were 2–4 °C warmer than average for 10 weeks. Its kelp forest, usually 800 kilometres long, shrank by 43%.
These fluctuations are confusing for marine-resource managers, policymakers and the public. They make decisions about how best to adapt to climate change difficult, and short term forecasts unreliable. …. “