Questions in the ‘Global Warming: Controlling’ Category

Are there natural processes that can amplify or limit global warming?

July 2, 2007

Yes. Changes in one part of the climate system trigger processes that may either amplify the initial change or counteract it. With a positive climate feedback, warming triggers changes that cause more warming. With a negative climate feedback, warming triggers changes that lead to cooling.

The most fundamental negative (cooling) feedback is that the Earth radiates heat into space based on its temperature. The relationship between temperature and radiated heat is such that an increase in temperature is accompanied by an even bigger increase in radiated heat. The feedback does not prevent temperature from rising, but it slows the rate of temperature increase (or decrease) that a given energy imbalance can cause. The radiative feedback allows the Earth to achieve a new balanced (equilibrated) state in response to a change in surface temperature.

The other key feedbacks are water vapor, snow and ice, and clouds. Warming temperatures increase the amount of water vapor in the atmosphere. Because water vapor is a powerful greenhouse gas, it amplifies warming. Decreases in snow and ice make the Earth less reflective to incoming sunlight, also amplifying warming. Changes in clouds may either amplify or limit global warming, depending on where (latitude and altitude) and when (time of year) changes occur. Nearly all climate models scientists use today predict that net cloud feedbacks will either be neutral or positive (warming), but such predictions are still uncertain.

Numerous other feedbacks also exist. Warmer temperatures may decrease the rate at which the ocean absorbs carbon dioxide. Global currents that distribute heat among the world’s oceans may change because of temperature and salinity changes. Expansion or contractions of global vegetation can influence the reflection and absorption of incoming sunlight, the flow of energy and moisture between the surface and the air, and the carbon cycle. With the exception of not knowing precisely how much humans will do to control greenhouse gas emissions in coming decades, the strength of climate feedbacks—especially cloud feedbacks—is the biggest source of uncertainty in predictions of future climate.

  1. References

  2. Bony, S., Colman, R., Kattsov, V., Allan, R., Bretherton, C., Dufresne, J., Hall, A., Hallegatte, S., Holland, M., Ingram, W., Randall, D., Soden, B., Tselioudis, G., and Webb, M. (2006). How well do we understand and evaluate climate change feedback processes? Journal of Climate, 19(15), 3345-3482. doi: 10.1175/JCLI3819.1.
  3. Soden, B., and Held, I. (2006). An assessment of climate feedbacks in coupled ocean-atmosphere models. Journal of Climate, 19(14), 3354-3360. doi: 10.1175/JCLI3799.1

If we immediately stopped emitting greenhouses gases, would global warming stop?

July 2, 2007

Not right away. The Earth’s surface temperature does not react instantaneously to the energy imbalance created by rising carbon dioxide levels. This delayed reaction occurs because a great deal of the excess energy is stored in the ocean, which has a tremendous heat capacity. Because of this lag (which scientists call “thermal inertia”), even the 0.6–0.9 degrees of global warming we have observed in the past century is not the full amount of warming we can expect from the greenhouse gases we have already emitted. Even if all emissions were to stop today, the Earth’s average surface temperature would climb another 0.6 degrees or so over the next several decades before temperatures stopped rising.

The time lag is one reason why there is a risk in waiting to control greenhouse gas emissions until global warming becomes worse or its effects more serious and obvious. If we wait until we feel the amount or impact of global warming has reached an intolerable level, we will not be able to “hold the line” at that point; some further warming will be unavoidable.

  1. References

  2. Hansen, J., Sato, Mki., Ruedy, R., Kharecha, P., Lacis, A., Miller, R.L., Nazarenko, L., Lo, K., Schmidt, G.A., Russell, G., Aleinov, I., Bauer, S., Baum, E., Cairns, B., Canuto, V., Chandler, M., Cheng, Y., Cohen, A., Del Genio, A., Faluvegi, G., Fleming, E., Friend, A., Hall, T., Jackman, C., Jonas, J., Kelley, M., Kiang, N.Y., Koch, D., Labow, G., Lerner, J., Menon, S., Novakov, T., Oinas, V., Perlwitz, Ja., Perlwitz, Ju., Rind, D., Romanou, A., Schmunk, R., Shindell, D., Stone, P., Sun, S., Streets, D., Tausnev, N., Thresher, D., Unger, N., Yao, M., and Zhang, S (2007). Dangerous human-made interference with climate: A GISS modelE study. Atmospheric Chemistry and Physics, 7, 2287-2312.
  3. Hansen, J., Nazarenko, L., Ruedy, R., Sato, Mki., Willis, J., Del Genio, A., Koch, D., Lacis, A., Lo, K., Menon, S., Novakov, T., Perlwitz, J., Russell, G., Schmidt, G.A., and Tausnev, N. (2005). Earth’s energy imbalance: Confirmation and implications. Science, 308, 1431-1435. doi: 10.1126/science.1110252

If we stabilized greenhouse gas emissions at today's rates, would global warming stop?

July 2, 2007

No. Carbon dioxide levels are rising because we currently emit more carbon dioxide into the atmosphere than natural processes like photosynthesis and absorption into the oceans can remove. Therefore, stabilizing emissions at today’s rates will not stop global warming: our carbon dioxide “deposits” would still exceed natural “withdrawals.” Atmospheric carbon dioxide levels would continue to increase, and temperatures would continue to rise. To stop global warming, we will have to significantly reduce not just stabilize, emissions in coming decades.

CO2 Concentration Scenarios

Since the start of the industrial revolution, humans have been burning fossil fuels and emitting carbon dioxide faster than natural processes can take it back up. As a result, concentration of carbon dioxide has been rising (black line segment.) The graph above shows how different scenarios would affect carbon dioxide concentrations. To reduce the amount of carbon dioxide in the atmosphere (purple), emissions have to fall below the amount that plants and the ocean absorb. Smaller reductions (blue line) or capping emissions at current rates (green line) will only slow the rate of growth. (Graph by Robert Simmon, NASA Earth Observatory.)

  1. References

  2. U.S. Department of Energy, Energy Information Administration. (2004, April 2). Greenhouse Gases, Climate Change, and Energy. Accessed June 29, 2007.
  3. Intergovernmental Panel on Climate Change. (2007). Chapter 10: Global Climate Projections 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, United Kingdom, and New York, New York: Cambridge University Press.

Climate Q&A

Share