Module Summary


In Module 4, we discussed the very strong scientific evidence that our burning of fossil fuels is raising atmospheric CO2, with an unavoidable warming influence on the climate. Temperatures are in fact rising, despite the cooling effect of recent slight dimming of the sun, blocking of the sun by particles from our smokestacks, and our actions in cutting dark forests to replace them with grasslands that reflect more sunshine. The success of climate models in explaining what has happened, “retrodicting” history by starting in the past and running toward the present, and the very clear evidence that climate is doing what earlier climate scientists projected, give us high confidence that our scientific understanding is correct. And, considering how much fossil fuel remains in the ground, we have high confidence that if we continue to burn rapidly, the coming changes will be large compared to those that have happened so far.

People typically are most interested in how climate will affect them—global mean surface temperature is rarely as interesting as dinner, and whether or not dinner will be available. Looking at a great range of scholarship, small climate changes tend to cause winners and losers. Generally, poor people in hot places are hurt by a little warming, whereas wealthier people in colder places are not impacted as much and may even benefit slightly. But, as the climate changes become larger, the losers grow to far outnumber winners.

The biggest concern may be that many of our crops are already damaged by excessive heat, but by late in this century if we continue burning fossil fuels rapidly, much of the world’s cropland is likely to see average temperatures hotter than the hottest ever experienced so far. If the climate is favorable, plants grow better with more CO2 in the air, but the damages from higher temperatures are expected to grow to greatly exceed the benefits of this CO2 fertilization, made worse by increasing floods and droughts, and by invasive pests. Other impacts of climate change are also expected to hurt more than help for humans and most other species.

This is real science, so there are real uncertainties. But, this is not reassuring to most people who look carefully. The uncertainties are generally on the “bad” side—things may be a little better or a little worse, but with almost no chance of being a lot better but some chance of being a lot worse. Building almost anything requires getting many things right, but breaking can be done with a big hammer or a stick of dynamite. By analogy, adding CO2 to the air is very unlikely to create paradise, but might greatly damage many things that we care about.

In case you find this scary or depressing, please stay with us. The next Unit of the course covers the amazing resources that are available to us, with the potential to power everyone on the planet almost forever. And in the third Unit, we discuss how the use of this knowledge can make us better off, with a bigger economy, more jobs, greater national security, and a cleaner environment where we treat each other more fairly. Fossil fuels have given us another step on the ladder to a better future, and while they cannot get us to the top, other sources of energy really can. 

Reminder - Complete all of the Module 5 tasks!

You have reached the end of Module 5! Double-check the Module Roadmap to make sure you have completed all of the activities listed there before you begin Module 3.

References and further reading

You can find the key results for this, and other modules in the reports of the IPCC, and of the US National Academy of Sciences, and in Dr. Alley’s book Earth: The Operators’ Manual (it isn’t free, though); a quick look by Dr. Alley indicates that most of the Wikipedia pages are pretty good, too. A few of the numbers in Modules 4 and 5 may be harder to find though and those references are given here.

  1. Estimates of food stress for future summers hotter than any seen up to 2006 are from Battisti, D.S. and R.L. Naylor, 2009, Historical warnings of future food insecurity with unprecedented seasonal heat, Science 323, 240-244.
  2. Estimated losses of US corn and soybean production with warming are from Schlenker, W. and M.J. Roberts, 2009, Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change, Proceedings of the National Academy of Sciences of the United States of America 106, 15994-15998.
  3. The death toll from the 2003 Europe heat wave is estimated in Robine, Jean-Marie; Siu Lan K. Cheung, Sophie Le Roy, Herman Van Oyen, Clare Griffiths, Jean-Pierre Michel, François Richard Herrmann, 2008, Death toll exceeded 70,000 in Europe during the summer of 2003, Comptes Rendus Biologies 331, 171-178.
  4. The reductions in work outside were estimated by Dunne, J.P., R.J. Stouffer and J.G. John, 2013, Reductions in labour capacity from heat stress under climate warming, Nature Climate Change 3, 563-566.
  5. IPCC, 2007: Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working
    Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning,
    Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Note: Dr. Alley was one of the Drafting Authors for this report.
  6. Schmidt, G.A., R. Ruedy, R.L. Miller, and A.A. Lacis, 2010: The attribution of the present-day total greenhouse effect. J. Geophys. Res., 115, D20106, doi:10.1029/2010JD014287.
  7. For more from the IPCC, including all of the 2013 report, see IPCC. Especially look at Climate Change 2013: Physical Science Basis , and start with Summary for Policymakers (SPM) , focusing in on IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY.  The basic information here was mostly in place in the Fourth Assessment from 2007, and really was fairly well know in the decades before that, but the science continues to get better, and the IPCC has continued to update to give you the latest and greatest.