I this week attended the European Commission’s stakeholder conference on the 2015 climate agreement. It didn’t yield much news on the 2015 issue, but got kicked off with an excellent overview of the current state of climate science by Prof. Schellnhuber from the Potsdam Institute for Climate Impact Research, a little preview of the IPCC’s next assessment report that will come out this year and next. The whole event can be watched on video here, Schellnhuber starts a few minutes into the video, his presentation is about half an hour long. Below are some notes I took.
Schellnhuber argued that while the media often give the impression that the state of science is a state of confusion (“Man-made climate change is a hoax – and nothing in the world can stop it!”), actually the science is complex but not controversial, not complete but compelling.
The record from ice cores, sediments etc. shows that humankind has had 11,000 years of grace, the Holocene. In the preceding tens of thousands of years global average surface temperature moved up and down tremendously, but over the last 11,000 years it became extremely stable. A reconstruction of the last 10,000 years shows that there was a temperature maximum mid-term into the Holocene, then a certain drop of some tenths of degrees, and then the spike starting at the beginning of industrialisation. The previous cooling had to do with orbital changes, humanity actually escaped the next ice age very narrowly. If solar insulation had dropped just 1% more per cent, we would have had the next ice age.
Warming of average surface temperature has indeed slowed in the last 15 years, but this doesn’t mean that warming has stopped as climate contrarians like to claim. “Surface temperature” is the critical term here, the slowdown is due to a decade of La Niña events. While El Niño events release heat from the ocean to the atmosphere, La Niña events transfer heat from the atmosphere into the oceans.
We can measure from satellites that Earth receives more energy from the sun than it radiates back to space. So there is a net uptake of energy by Earth, which is not in any way questioned. And according to the first law of thermodynamics, if you put energy into a system, it has to go somewhere. 1% is going into melting ice, most of it is going into the ocean. The top 3 metres of the ocean have more heat content than the entire atmosphere. The ocean is thousands of metres deep. So clearly anything that happens in the ocean can have big impacts on the surface. There has been a tremendous uptake of ocean heat since just before the slowdown of surface warming around the year 2002, the biggest ocean heat uptake ever recorded.
We might even have another decade of slow surface warming if the intermediate heat in the ocean goes to the deep sea, but then it will come back.
And if the oceans warm like this, it means increasing sea-level rise, and that’s what we see in the satellite data, sea-level rise has accelerated.
Contrary to what some recent news articles claimed, climate scientists are not revising their estimate of climate sensitivity, the amount of warming that is caused by a doubling of the atmospheric CO2 concentration. Climate sensitivity is an asymptotic concept, it refers to what happens in equilibrium after 500 or 1,000 years. So it is completely ridiculous to revise climate sensitivity after 10 years of slow warming. A recent Nature article based on the last 65 million years suggests a sensitivity of 2.2-4.8° per doubling of CO2, which agrees with IPCC estimates. The best estimate of the IPCC remains at 3°.
Business as usual will see a trebling of atmospheric GHG concentration and 6° temperature increase by the end of this century, 8° by 2300. Meeting the 2°C target will require strong action. Representative concentration pathway (RCP) scenario RCP3-D from the next IPCC report scratches 2° around 2070 before temperature starts going down again. The scenario envisages a peak of global emissions by 2020, a reduction of emissions to zero by 2070 and negative emissions afterwards through tree planting and other measures.
The business as usual scenario assumes a linear development but as the planet heats up it is likely that tipping points will be reached. We will see a melting of ice sheets, a destruction of coral reefs, destabilisation of the Monsoons etc. A warming of 1.6°C would lead to an irreversible melt of the Greenland ice sheet – something to be appreciated in the debate about 2° –, but it could take 10,000 years. With 8° warming, it could take only centuries. Bad news from Antarctica from just several days ago, there is an unprecedented melting as well, probably due to the redistribution of heat in the oceans. In order to save just 10% of coral reefs, we would need to stabilise at 1.5°C, due to warming, sea-level rise and ocean acidification. The fate of the Amazon is unclear. Some models predict that it will turn into savannah, others say that it will be stable. They all hinge on the effect of CO2 fertilisation, which is difficult to nail down. The current assumption is that fertilisation will be dominant over the next 100 years and compensate for increased climate stress through droughts etc., but we cannot be sure.
The recent report “Turn Down the Heat” on behalf of the World Bank (which was downloaded 50 million times) shows that climate protection is the only way to safeguard development in the South. The tropics will not only have higher warming, they will also get above-average sea-level rise. The sea level around Greenland and West Antarctica will actually decrease because the thousands of meters of ice layers melting means losing mass, which means losing gravitational pull. So the water is just released and flows away, to the tropics.
There will also be a drastic increase of extreme heat waves. The report calculated 5-sigma events, 5 sigma means five standard deviations from the mean, which under current climate conditions happens once in a million years. In a world that is 4° warmer, it will happen every second year in many places in the tropics and sub-tropics. Africa will have 3 billion people by 2100, their daily climate will be pushed out five standard deviations from the current situation. “Those who say adaptation will be cheaper than mitigation, please pay for the costs of operating an agricultural system under these conditions.”
Indian summer monsoon will on average be increased but will become more volatile, there may be a three years in a row without monsoon, while 60% of Indians make their livelihood in agriculture.
The news for the North is that the jet stream, which dominates weather in the North, is becoming more wrinkled. In general the jet stream is very straight, if there is a wave to the North or South, it goes away after a few days. In the future, waves and the related weather systems may increasingly get stuck for weeks. This is precisely what happened with the Russian heat wave in 2010, which was stuck for six weeks, and at the same time there was a cool wave to the east and south, which caused the record flood in Pakistan. This change of the jet stream may be the most vicious of all tipping elements.
The next IPCC report will have a world-first analysis of low stabilisation scenarios and related costs. This will be the key result of the next report, there will not be much new in the reports of working groups 1 and 2. The analysis considers that achieving the 2°C target is achievable at costs of around 1% of GDP. But the scenarios only work if global emissions peak by 2020. Every year of delay costs hundreds of billions of dollars.