Little time left to act on climate change Copenhagen: After this weekend's climate conference in Copenhagen, Fairfax Media sat down with three of the Intergovernmental Panel on Climate Change's leading experts - two of them Australian - to ask them about the latest science, and the options and challenges facing the world. The ocean expert: "You can look at sea levels and say 'gee, this is a depressing story'. But you can also say there's an opportunity to change it. "We have the means to limit climate change": Intergovernmental Panel on Climate Change chairman Rajendra Pachauri. Quiet-voiced, bushy-bearded Dr John Church, who works in Tasmania for the CSIRO, is one of the world's most respected experts on sea levels. He was picked to be one of the leading authors for the IPCC's climate reports. "It's very clear the sea level has been rising during the 20th century, faster than during the pre-industrial period," he said "I argue there has been an acceleration during the 20th century." This latest IPCC report is the first time scientists can explain to their own satisfaction why sea levels are rising, Dr Church said. Oceans are expanding as they heat up, glaciers are melting, and in the last few decades we have been pumping water out of aquifers faster than we are storing it in dams. The correlation is clear between hotter climate and higher sea levels, Dr Church explained. Thousands of years in the past, between ice ages, when the global temperature was about 2 degrees warmer than today, sea levels were 5-10 metres higher than today. Elsewhere in the IPCC report, experts predict that even with a fairly strong level of action on greenhouse gas emissions in the next couple of decades, by the end of the century the world will be about 2 degrees warmer. Dr Church said the biggest risk of a dramatic, unstoppable rise in sea levels lies in the huge ice sheet on Greenland. At some stage there will be a "tipping point," after which the ice sheet melts faster than it can gather snow and ice. Add that water to the oceans, and the sea level will eventually be 7 metres higher. "We estimate that threshold to be somewhere between 1 degree Celsius to 4 degrees above pre-industrial temperatures," Dr Church said. The IPCC's report estimates we are already 0.85 degrees above pre-industrial temperatures, and we are "locked in" to reach 1.1 degrees by 2035. "So there's a very good chance that without very significant, urgent and sustained mitigation this century, we will cross that threshold for the Greenland ice sheet [melting]," Dr Church said. Dr Church will not take part in the next IPCC round - "I found this very tough this time," he said. For me this started in July 2009, it's been a five-year-plus journey and it's taken a huge effort over that period. Long days and long nights. Asked what would make the work worthwhile, Dr Church said it would be seeing the world switch to renewable, low-carbon energy. "Australia has made significant progress," he said. I think now we should consider how we should increase that renewable energy target. By 2040 Australia should aim at 40 per cent of our energy coming from renewable sources, he said. But no one nation can solve this - only by pulling together can we solve the problem, he said. The climate expert: If we were able to magically stop greenhouse gas emissions today, the planet will stay warm for centuries. "We have already committed ourselves," said Dr Scott Power, IPCC author and a senior researcher at the Bureau of Meteorology. Our prediction is that the temperature of the planet will be something like 1.1 degrees warmer than the latter half of the 19th century [by 2035]. That's the best expectation. After 2040, it's up to us whether we keep on warming to 4 degrees and beyond, or keep to a 1.5-2 degree bump. And that depends pretty much on how much greenhouse gas we put in the atmosphere from now on. Under the business as usual scenario, Australia is up for a shopping list of undesirables: loss of the Great Barrier Reef ecosystem from heat and acidification, more days of "severe fire weather" each year, more extreme rainfall and resulting flooding, less drinking water, and more crop-killing heatwaves. Dr Power said the so-called recent "hiatus" in global warming has been explained by several new studies - confirming the conclusions in the report. Though the average surface temperature has not risen significantly in the last decade or so, this is because heat has been dragged into the depths of the ocean more quickly than usual. "It's not true to say the climate system wasn't warming up," Dr Power said. But this was an aberration due to short-term natural variability. Such variability could just as easily speed up global surface temperature change. Another short-term difference has been unusually high volcanic action, which reflected a bit more heat than usual back into space - and the output of the sun also showed a little dip over the same period. "It doesn't really change any of the major conclusions [of the IPCC's reports]," he said. The economics expert: "We have a choice." It's a choice as to how quickly we act to head off climate change, and how much we reduce our carbon emissions over the coming decades, said German climate economist Professor Ottmar Edenhofer. It is like an overweight person who is worried about their long-term health, he explained. You can say, I will become a little fatter now then lose weight in 10 years, more rapidly. Is that a reasonable option? I would say if you want to lose weight you have to start with something. You have to go to the gym, maybe not everyday, maybe three days a week. What I am saying is we shouldn't lose too much time to focus on the long-term stuff. Countries need to start with a carbon price, he said - preferably within the next decade. You can do it by a carbon tax or emissions trading but nevertheless you have to do this. Countries have to learn it does not destroy their economies.