I am writing my blog this week from the American Association for the Advancement of Science meeting in Washington DC, where I’ve just been talking about the implications of climate tipping points for policy and societies. It was a packed session on tipping points in ‘social-ecological systems’ and there were many audience questions about climate change. So, what better time to answer some of your burning questions that we didn’t get to in this week’s feedback video…
Is it misleading to start recent temperature change graphs from 1850 as this is when the Little Ice Age ended? Surely we’d expect a temperature rise then?
I don’t think it’s misleading. The observational temperature record starts when there is sufficient coverage of thermometer measurements to reasonably accurately reconstruct northern hemispheric temperature. It happens that this was roughly half way through the nineteenth century, around the end of the Little Ice Age. But this is a coincidence not a deliberate effort to show a temperature rise. It’s also important to note that the Little ‘Ice Age’ was a regional phenomenon – there was a marked cooling in Western Europe and the North Atlantic region – but other parts of the planet didn’t cool. The warming observed now is global and most of it has occurred since about 1950 – what goes on between 1850 and 1950 does not make a big difference to the figures.
Why are temperature anomalies recorded relative to the 1961-1990 average?
It was not until this 30 year interval that we could make a really precise estimate of global temperature – thanks to an expansion of surface measuring stations. The temperature in earlier intervals is less precisely known because there were fewer weather stations.
If we have more CO2 in the atmosphere, doesn’t this mean more photosynthesis (as CO2 is a limiting factor) and a negative feedback begin?
Yes it does, and this response – called the ‘CO2 fertilisation effect’ – is a key reason why there is a land carbon sink – i.e. a net land uptake of CO2 of around 2.5 billion tonnes of carbon per year. This is indeed a negative feedback that is slowing the rate of rise of CO2 concentration in the atmosphere, and the corresponding global warming.
Is there much hope after the COP21 event that countries such as the US and China will begin reducing their emissions?
In their ‘INDCs’ (intended nationally determined contributions) China do not commit to reducing their emissions for some time – instead they commit to lowering the amount of carbon they emit per unit of GDP (but growth of GDP is expected to outweigh this for some time). The US have voluntarily commit to reducing their greenhouse gas emissions by 26-28% from 2005 levels by 2025. US emissions have already been declining slightly, but the decline will have to accelerate considerably to meet this target. The best hope globally for me is if solar energy technologies and other sustainable energy technology continue to drop in price. Then economics will compel a switch away from fossil fuel burning. But there is so much relatively cheap coal left in the ground that what nations really need to wrestle with is an agreement to leave most of it underground (or we need a global price on carbon high enough that it compels us to capture and store carbon dioxide from all power stations where fossil fuels are burned).