Much of the changes humans are causing in our atmosphere today will be largely irreversible for the rest of the millennium.

WHO: Susan Solomon, Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA
Gian-Kasper Plattner, Institute of Biogeochemistry and Pollutant Dynamics, Zurich, Switzerland
Reto Knutti, Institute for Atmospheric and Climate Science, Zurich, Switzerland,
Pierre Friedlingstein, Institut Pierre Simon Laplace/Laboratoire des Sciences du Climat et de  l’Environnement, Unité Mixte de Recherche à l’Energie Atomique – Centre National de la Recherche Scientifique–Université Versailles Saint-Quentin, Commissariat a l’Energie Atomique-Saclay, l’Orme des Merisiers, France

WHAT: Looking at the long term effects of climate pollution to the year 3000

WHEN: 10 February 2009

WHERE: Proceedings of the National Academy of Sciences of the USA (PNAS), vol. 106, no. 6 (2009)

TITLE: Irreversible climate change due to carbon dioxide emissions

Stopping climate change often involves the metaphor of ‘turning down the thermostat’ of the heater in your house; the heater gets left on too high for too long, you turn the thermostat back down, the room cools down, we are all happy.

This seems to also be the way many people think about climate change – we’ve put too much carbon pollution in the atmosphere for too long, so all we need to do is stop it, and the carbon dioxide will disappear like fog burning off in the morning.

Except it won’t. This paper, which is from 2009 but I came across it recently while reading around the internet, looks at the long term effects of climate change and found that for CO₂ emissions, the effects can still be felt for 1,000 years after we stop polluting. Bummer. So much for that last minute carbon detox that politicians seem to be betting on. Turns out it won’t do much.

The researchers defined ‘irreversible’ in this paper at 1,000 years to just beyond the year 3000, because over a human life span, 1,000 years is more than 10 generations. Geologically, it’s not forever, but from our human point of view it pretty much is forever.

So what’s going to keep happening because we can’t give up fossil fuels today that your great-great-great-great-great-great-great-great-great-great grandkid is going to look back on and say ‘well that was stupid’?

The paper looked at the three most detailed and well known effects: atmospheric temperatures, precipitation patterns and sea level rise. Other long term impacts will be felt through Arctic sea ice melt, flooding and heavy rainfall, permafrost melt, hurricanes and the loss of glaciers and snowpack. However, the impacts with the most detailed models and greatest body of research were the ones chosen for this paper (which also excluded the potential for geo-engineering because it’s still very uncertain and unknown).

Our first problem is going to be temperature increases, because temperatures increase with increased CO₂  accumulation in the atmosphere, but if we turned off emissions completely (which is unfeasible practically, but works best to model the long term effects) temperatures would remain constant within about 0.5^oC until the year 3000.

Why does this occur? Why does the temperature not go back down just as quickly once we stop feeding it more CO₂? Because CO₂ stays in the atmosphere for a much longer time than other greenhouse gases. As the paper says: ‘current emissions of major non-carbon dioxide greenhouse gases such as methane or nitrous oxide are significant for climate change in the next few decades or century, but these gases do not persist over time in the same way as carbon dioxide.’

Our next problem is changing precipitation patterns, which can be described by the Clausius-Clapeyron law of the physics of phase transition in matter. What the law tells us is that as temperature increases, there is an increase in atmospheric water vapour, which changes how the vapour is transported through the atmosphere, changing the hydrological cycle.

The paper notes that these patterns are already happening consistent with the models for the Southwest of the USA and the Mediterranean. They found that dry seasons will become approx. 10% dryer for each degree of warming, and the Southwest of the USA is expected to be approx. 10% dryer with 2^oC of global warming. As a comparison, the Dust Bowl of the 1930s was 10% dryer over two decades. Given that many climate scientists (and the World Bank) think that we’ve already reached the point where 2^oC of warming is inevitable, it seems like Arizona is going to become a pretty uncomfortable place to live.

Additionally, if we managed to peak at 450ppm of CO₂, irreversible decreases in precipitation of ~8-10% in the dry season would be expected in large areas of Europe, Western Australia and North America.

Finally, the paper looked at sea level rise, which is a triple-whammy. The first issue is that warming causes colder water to expand (aka thermal expansion) which increases sea level. The second is that ocean mixing through currents will continue, which will continue the warming and the thermal expansion. Thirdly, warming of icecaps on land contributes new volume to the ocean.

The paper estimates that the eventual sea level rise from thermal expansion of warming water is 20 – 60cm per degree of climate change. Additionally, the loss of glaciers and small icecaps will give us ~20 – 70cm of sea level rise too, so we’re looking at 40 – 130cm of sea level rise even before we start counting Greenland (which is melting faster than most estimates anyway).

What does all of this mean? Well firstly it means you should check how far above sea level your house is and you may want to hold off on that ski cabin with all the melting snowpack as well.

More importantly though, it means that any last minute ‘saves the day’ Hollywood-style plans for reversing climate change as the proverbial clock counts down to zero are misguided and wrong. The climate pollution that we are spewing into the atmosphere at ever greater rates today will continue to be a carbon hangover for humanity for the next 1000 years or so. Within human time scales, the changes that we are causing to our atmosphere are irreversible.

So naturally, we should stop burning carbon now.