Why our Earth system is warming

DR PEP CANADELL: Climate change it is like a global problem in the sense that the atmosphere is a commons for society. What we Australians do or somebody else does at the other end of the world, it actually all gets mixed within a year. So we really need to develop strategies, which are strategies working globally in partnership. Both in the science to understand it, but also in the way we address the mitigation ultimately. 

DR JOHN CHURCH: Well, the Earth is warming, principally, because we’re releasing greenhouse gases into the atmosphere. These trap the outgoing long wave radiation and lead to a warming Earth. The energy, total energy in the Earth’s system increases and much of that energy is locked up in the oceans. 

DR STEVE RINTOUL: I’m an oceanographer and the oceans are important to climate because they store huge amounts of heat and carbon dioxide. And in fact about 93 per cent of the extra heat that’s been stored by the Earth over the last 50 years, is found in the ocean. So I, we want to understand climate change and track how it’s evolving we really need to be measuring the ocean. 

And that’s relevant to current discussions about the rate at which the Earth has warmed recently. So, over the last decade or so, the speed at which the surface has been warming has slowed a bit, it’s still warming, and the last decade is still the warmest decade in the instrumental record. But the rate of warming has slowed, even though the amount of CO₂ in the atmosphere has increased, so that sounds like a surprise at first, but we’re beginning to understand exactly how that’s happening. 

DR JOHN CHURCH: We also see other changes; glaciers are continuing to lose mass. The sea ice, particularly in the Arctic, is continuing to retreat and the ice sheets of Greenland and Antarctica are contributing by losing mass contributing to sea level rise, which is continuing to rise at a faster rate over the last 20 years than over the 20th century as a whole. 

DR STEVE RINTOUL: These pauses or periods when the Earth warms more slowly than average are not unusual. They happen in the climate system frequently. The second point is that we now understand what’s happening and part of what’s happening is that the heat in the Earth’s system is moving around and, in particular, more of the heat that used to be near the surface of the ocean has moved deeper in the ocean so if we look at the … how the heat that’s stored by the Earth has changed over the last decade and include the upper 2,000 metres of the ocean, we see that the Earth has continued to warm during this last decade just like it has in the decades before. 

The other factor that contributes is that the Sun that goes through a cycle of about a decade of energy output from the Sun is increasing and then decreasing has been in a decreasing phase over the last 10 years and that’s now turning around. And finally there have been a few small volcanoes and volcanoes put small particles up into the atmosphere and that tends to shield the Earth from some of the Sun’s energy and that also causes the Earth to cool. So the combination of those factors can explain why the surface of the Earth both on land and in the ocean has warmed less rapidly over the last decade than it did in the decade before. 

DR PEP CANADELL: So greenhouse gases are gases which are naturally occurring in the atmosphere. They trap some of the energy from the Sun and allows the Earth to be a very comfortable place as a whole to live in, so a lot of the human activities specifically the combustion of fossil fuels, coal, gas and oils and also what we call land use change deforestation, largely the burning of the forest, are the main causes of greenhouse gases. So we have now acceleration in the accumulation of these greenhouse gases and that’s why there is a warming and warming actually leads to other changes in climate patterns including precipitation and others. 

DR JOHN CHURCH: To predict future changes just extrapolating the past is not sufficient. What we have to do is build an understanding of what’s happened in the past, put this understanding in climate models which could then simulate the past, test them against the past and then use those climate models with scenarios of how our society might behave in the future in terms of emitting greenhouse gases and run those models into the future to predict future conditions. 

DR STEVE RINTOUL: So to study the ocean, to actually measure how the ocean’s changing, we use a variety of tools. We use ships that we lower instruments from, ships that scan the ocean, use satellites, use instruments that we anchor to the sea floor for a year or two and we use these days simple robots, floats that drift through the ocean current at a depth of 1 kilometre. Every 10 days they drop down to 2 kilometres and then rise up to the surface measuring temperature and salinity. We transfer that data by satellite, sink back down and do it all again and do that for five years or so for each float. There are about 3,600 of these floats now drifting around the world oceans and so in a real sense we’re measuring the oceans for the first time. 

DR PEP CANADELL: We put a huge amount of effort in taking all these measurements and expensive measurements and expensive development of modelling capability but also we’re putting an increasing amount of time in packaging this information in ways that can be readily used and understood and make a difference for the things that policy needs information on. 

DR STEVE RINTOUL: The last decade has been the warmest in the instrumental record and each of the last few decades has been warmer than the last and that’s part of the signal of greenhouse warming. The last 12 months for Australia has been the warmest on record and that is the combination of the slow continued warming due to greenhouse warming and the variability of the climate system and so we will get years that will be cooler than this last one but that’s what we expect to see. The climate …the warming of the Earth and the warming over Australia will not just be a steady trend, it’ll go up and down a little bit. But the trend over longer time periods which is what you need to look at in order to detect a signal of climate change has been up and will continue to be warming in the future. 

DR JOHN CHURCH: Sea levels are continuing to rise. The amount of rise depends on future emissions of greenhouse gases. Larger emissions lead to larger rises both during the 21st century and beyond so the amount of sea level rise that Australia will have to deal with is impacted by the degree to which we mitigate ours and the world’s emissions of greenhouse gases. We cannot stop all sea level rise. We will have to adapt, but the amount of adaptation that we have to do will depend on our future emissions. 

DR STEVE RINTOUL: So, the challenge for climate science in the future, it’s clear that the Earth is warming and it’s clear that human activities have contributed. What is still more difficult to do is to project regional changes of temperature and, in particular, precipitation and that’s crucial because the decisions that society will make to adapt to the climate change that we don’t avoid requires information at those scales, at local and regional scales because that’s where people make decisions. And so a challenge for the climate science is to deliver that sort of information, regional projections of changes in climate that can inform decision-makers about both how hard we’re going to work to slow down the rate of climate change by mitigating, by reducing greenhouse gas emissions and how we can most effectively adapt to the climate change that we’re not going to avoid.