A Skeptic's Guide to Global Warming

Isn't climate always changing?

Yes, climate is always changing. However, just because climate can change due to natural causes doesn't mean that it can't also change from human-made causes.

Geological evidence (figure, from AR4) shows dramatic changes over the last half a million years. Ice caps (bottom curve in figure) covered much of North America during long ice ages except during interglacials (marked in gray on figure) such as the last 10,000 years. Global mean temperature may have differed by only about 5^oC between ice ages and interglacials.

Note the time axis: each pair of tick marks spans 20,000 years. The changes shown in the graph occur over much longer times than a century. They are driven by features of the Earth's orbit and ice caps that can not explain the warming of the last 100 years. Neither can the mechanisms of other big climate changes over the lifespan of the Earth.

CO₂ concentration (red curve in figure) also varied between ice ages and interglacials. While not a cause of those climate changes, it is probably an amplifier. If growing ice caps drive other changes which lower CO₂ concentration, the reduction in greenhouse effect will further cool the planet. This may be part of the reason that slight changes in the Earth's orbit drive big changes in climate. It is also consistent with CO₂ being an important element of climate variation.

There has been tremendous controversy over whether the 1^oC global mean surface warming of the last century is unusual compared to other periods in the last 2000 years. Estimates of temperatures from before the late 19th century are extremely uncertain. The latest IPCC report says that "there is medium confidence that the last 30 years were likely the warmest 30-year period of the last 1400 years" (AR5, Sec 5.3.5.1, last paragraph). If the recent warming is unusual, it strengthens the case for humans being the cause. If not, it is still possible that other mechanisms which do not explain recent warming, such as solar forcing, drove variations over the previous 20 centuries.

Next: Have past climate predictions been correct?
Return to front page.

References

AR4: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007 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.

AR5: Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, 2013, Stocker, T. F., D. Qin (eds.).

Last modified: 22 July 2018