Climate Change … the 350ppm imperative

Stabilising the Climate

NASA climate scientist James Hansen has spent most of the last 40 years studying our climate system. His understanding of the science is both broad and deep. Such scientists rarely write readable books for the public, they prefer the dense detail of scientific papers for an audience of other scientists. Hansen is one of the few who has written a very readable book (he has even included a little science fiction chapter at the end!) which explains exactly why we need to reduce carbon dioxide and other greenhouse gases and in particular why we must reduce carbon dioxide levels from the current level of 390 ppm to 350 ppm. The book is Storms of my Grandchildren.

What follows is a summary of the science.

There are three planks in Hansens scheme:

  1. A rapid phase out of coal fired power stations (unless all emissions are sequestered … this is unlikely). The phase out has to be such that no coal fired power stations are operating in 20 years time.
  2. A reversal of the last 200 years of deforestation. This will require a significant global dietary shift, because livestock drive deforestation and it is only by scaling back extensive livestock production that the earth can be reforested.
  3. A reduction in anthropogenic emissions of trace gases such as methane and nitrous oxide together with a steep reduction in black carbon production.  These gases and the black carbon aerosol are also a product of the livestock industry … but not exclusively. Other potent sources of methane are leaky natural gas pipelines and emissions from coal mines.

Without items 2 and 3, the phase out of coal fired power stations isn’t enough to stabilise the climate, but without the coal phaseout, the next few hundred years will see the earth’s natural systems totally unable to support anything like the current human population and sea level rise of 15 meters will wipe out many of the world’s major cities.

The evidence

We already saw how scientists know we have a large energy imbalance with more energy arriving at the planet than leaving, but how much will the planet heat up? Will the planet’s 3 massive ice sheets of Greenland, West Arctarctic and the main Antarctic melt?

The evidence for Hansen’s estimate of the warming we are in for and his climate stabilisation strategy is contained in decades of work by climate scientists. What follows is the outline only. Note that Hansen’s estimate of the probable warming is in line with the general consensus amount climate scientists, we single him out because he has written one of the most readable accounts of what scientists know and how they know it.

  1. Prior to the industrial period, the earth’s climate was fairly stable for a few thousand years. A couple of periods of warming or cooling but reasonably stable.  We know the extent of the ice sheets and the composition of the greenhouse gases during this period from tiny bubbles trapped in the ice in ice sheets. Imagine something like an apple corer, but over 3 kilometers long and of a bigger diameter. Scientists have used such things to pull out long cores of ice. The layer of ice in every year is quite clear in these cores and the gas bubbles trapped in the cores can reveal both the greenhouse gas composition … and, surprisingly, the temperature.
  2. The period prior to the end of the last ice age, about 20,000 years ago was similarly quite stable. Huge ice sheets covered much of the northern hemisphere and were stable for thousands of years. We know the extent of that ice and the composition of the atmosphere during that period.
  3. These two points in time act like equations that enable us to determine the change in temperature we can expect with changes in forcings (greenhouse gases, aerosols and the like).
  4. The change in temperature derived in item 3 can be verified against data from ice-cores about temperature and greenhouse gas concentrations going back hundreds of thousands of years.

It’s important to understand point 4.  This is not climate modelling. Climate change sceptics love to criticise climate models but conveniently ignore the fact that it isn’t ONLY models which predict the changes we are experiencing. The method just sketched is very direct and confirms the findings of the models. What models CANNOT yet do is to predict weather and regional climate changes … changes over small areas of the planet. These are the changes due to all that extra energy sloshing around. But the big things, the global average surface temperature are well predicted by over a dozen models, all built independently by teams of climate scientists. There are teams in Australia, Germany, Britain, the US, Canada, France, Italy and so on. All up, over 15 global climate models are used by scientists and they agree in the most important respects … climate change is real and we and our ancestors have caused it. The direct method outlined above backs the models.