2015 Week 3 – questions answered

Week 3 of our MOOC has thrown up lots of questions about observed changes in the climate and the carbon cycle. Here are some answers…

1) Could increased urbanization have an effect on global temperatures given the rate of increase and spread of this phenomenon?

It can have a very small effect – because the total urban area is a tiny fraction of the Earth’s surface. The more important issue is whether ‘urban heat island’ effects could bias the observational record of temperature change – by warming up weather stations that were once out in the countryside. The key point is this biasing effect was recognised by scientists and has been taken out of the compilations of the global temperature record.

2) Is thermal expansion of the oceans a long-term effect?

Yes, very long-term. The uptake of heat by the ocean will take thousands of years to complete, linked to the long overturning time of the deep ocean and the slow rate of diffusion of heat into the interior of the ocean. The thermal expansion is a product of that slow heating up. If it turns out that the Earth has a high ‘climate sensitivity’ this implies a very long timescale for heat uptake by the ocean to be complete (and ultimately a large temperature and sea level rise).

3) Can afforestation make a significant impact in reducing the impacts of climate change?

Afforestation has two key effects. It can store carbon in trees (and soils, depending on where the trees are planted), and it can alter the albedo (reflectivity) and other physical properties of the land surface. Taking up CO2 clearly reduces the impact of climate change, and I have estimated that globally afforestation may already be removing around 0.25 GtC/yr from the atmosphere. But this is small compared to 10 GtC/yr of emissions. And there is a problem – afforestation in the higher latitudes darkens the land surface (especially in winter, when the trees shade the snow) and this can cause them to warm the climate (more than they cool it by taking out CO2). In the future, the most optimistic scenario would be for afforestation to reverse the effects of historical deforestation, which has released about 150 GtC in total. But the regions where this was done would have to be chosen carefully to avoid the counterbalancing climate warming effects.

4) Will increasing Antarctic ice balance out Albedo lost from Arctic?

No. Although there has been some increase in Antarctic sea-ice linked to changing wind patterns in the Southern Ocean (in turn linked to the ozone hole and its interaction with the climate system), this is not set to continue in the longer-term. Meanwhile the Arctic sea-ice loss has accelerated and we are expecting largely ice-free summers in the Arctic ocean by roughly the 2030s.

5) What exactly is going on in the Antarctic? Some reports claim growth, others claim high rates of collapse + calving.

I am assuming this question is about the Antarctic ice sheet, not the sea-ice. Considering Antarctica as a whole, satellites that measure the effect of the ice sheet on the Earth’s gravity field find that it is shrinking overall. However, there are really two ice sheets – the East Antarctic ice sheet (larger) and the West Antarctic ice sheet (smaller) – which behave somewhat differently. Measurements of surface elevation (from e.g. high-precision laser altimeters) indicate that key parts of the West Antarctic ice sheet and parts of the edge of the East Antarctic ice sheet are losing altitude, i.e. losing ice into the ocean. This could already be irreversible for parts of the West Antarctic ice sheet which are grounded below sea level. However, the great bulk of the East Antarctic ice sheet is sat on a continent, and parts of its interior are gaining some mass due to extra snowfall. Still, we can be fairly sure from the gravity measurements that the overall balance is a loss of ice.

Keep up the good work!

Professor Tim

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