The latest climate change denial claim ahead of the release of the Intergovernmental Panel on Climate Change assessment report is that global warming stopped in 1998. A slowing in the observed rate of global warming has prompted dubious suggestions that temperatures have not increased significantly in the past 15 years.
It’s true that after rising rapidly in the 1990s, global average temperature increases at the earth’s surface have slowed since 1998. But warming hasn’t stopped.
The last decade was the hottest on record globally. Each year from 2000 to 2010, except 2008, was in the 10 warmest recorded globally.
What’s most concerning is that it should be considerably cooler than average, not hotter. Since 1997, several natural climate factors have aligned that should have produced a discernible cooling effect on global temperatures.
A lull in solar activity from 2005 to 2010, combined with two very strong La Niña episodes from 2010 to 2012, would be expected to produce a strong decrease in global temperatures.
Yet the world hasn’t cooled. On the contrary, global surface temperatures are moving in the opposite direction to natural climate variations, due to greenhouse gas warming.
To understand recent changes in the rate of heating, we need to look beyond surface temperatures. Warming at the earth’s surface is a narrow measure of climatic change, with only a fraction of greenhouse gas warming heating the two meters above the ground encompassed by surface thermometers.
Ocean heat content provides a more comprehensive measure of global warming, because the oceans are vast heat reservoirs. The oceans absorb about 93 per cent of the additional heat trapped in the atmosphere by greenhouse gases such as carbon dioxide.
Recent studies show that natural cooling in the Pacific Ocean has counteracted some of the warming effect of greenhouse gases as heat is pumped down into the deep ocean.
Changes in the Pacific Decadal Oscillation can help explain the slowing of warming at the surface. This is a natural ocean cycle that plays out over decades and has been in a cooling phase since 1998.
There have been several similar decades in the recent past when the rate of surface warming slowed. But after each of these ended, there was a marked ramp up in heating at the surface.
Greenhouse gas warming certainly won’t be linear, with the same increase in heat recorded in each successive decade. But the overall trend is clear – global warming hasn’t paused and the climate system continues to warm.
Dr Sophie Lewis is a research fellow in climate science, based the University of Melbourne and the ARC Centre of Excellence for Climate System Science. She is visiting The Age as part of a scientist-in-residence program organised by the Australian Science Media Centre.
Read article at the Sydney Morning Herald