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The subject of reconstructions of temperature variations of the past millennium has been discussed many times before on this site (see e.g. here, here, here, and here). Despite the apparent controversy, the basic conclusion–that the global and hemispheric-scale warmth of the past few decades appears anomalous in a very long-term context–has stood up remarkably well in many independent studies (see Figure 1).
Figure 1. Reconstructions of Northern Hemisphere temperatures for the last 1000 years (various colored curves) compared with instrumental record (black curve).
[source: Wikipedia] (click to enlarge)
This is not to say that all estimates agree in their details. Indeed, there is a fair scatter among the various published estimates. More »
Guest commentary from Ron Miller and Dorothy Koch (NASA GISS)
Scientists have confidence in a result to the extent that it can be derived by different investigators. Their confidence is increased if different techniques lead to the same conclusion. Concurrence provides evidence that the conclusion does not depend upon assumptions that occasionally are insufficiently supported. In contrast, two articles published last December on the same day arrive at very different and incompatible estimates of the effect of human-made aerosols on the radiative budget of the planet (Bellouin et al., 2005; Chung et al., 2005). They follow an earlier estimate published last year, (which included Dorothy as a co-author) that was in the middle (Yu et al., 2005). Aerosols are important to climate partly because their concentration is increased by the same industrial processes that increase the atmospheric concentration of greenhouse gases; yet aerosols generally oppose greenhouse warming. Because aerosols cause respiratory and other health problems and acid rain, they have been regulated more aggressively than greenhouse gases. Concentrations of some aerosols have decreased over the United States and Europe in recent decades as a result of environmental laws, although an increase has been observed in many thrid world regions, where economic development is a priority. In the twenty-first century, aerosol levels are anticipated to drop faster than greenhouse gases in response to future emission reductions, which will leave greenhouse warming unopposed and unmoderated.
Each published calculation of aerosol radiative forcing was a tour de force for integrating a wide variety of measurements ranging from absorption of radiation by individual particles to satellite estimates of aerosol amount. The disparate results emphasize the complexity and difficulty of the calculation. But let’s start at the beginning…. More »
The Wall Street Journal has published another fair and balanced critique of climate change science and negotiations, in a Business World commentary by Holman W. Jenkins, Jr, here. A summary of the arguments is as follows:
1. It will never be possible to prove that global warming is real. In the same way, we point out, it will never be possible to prove that anybody died from lung cancer because of smoking. Did you actually witness that first DNA mutation?
2. The reasonable lay person cannot be expected to read a scientific paper, so the rational response is to ignore the issue.
3. A paper about frogs did not argue convincingly that people cause global warming.
4. People sometimes distort the truth (truly a shocking charge coming from the WSJ).
5. Global change negotiations are stalled in politics, so the science must be wrong.
Final thought: When climate does change, we’ll be able to fix it anyway.
Living in central Pennsylvania, it would seem remiss of me not to comment on Groundhog Day today. For those not familiar with the event, Groundhog Day, which takes place on February 2 every year, is the modern American version of an age-old tradition originating in Europe centuries ago. The modern Groundhog Day is celebrated in the United States in Punksutawney Pennsylvania (about 100 miles west of Penn State University, where I teach). According to legend, if the groundhog–who is named Punxsutawney Phil–sees his shadow, there will be six more weeks of winter weather. If he does not see his shadow, there will be an early spring. After Phil emerges from his burrow on February 2, he speaks to an event official in “Groundhogese”, and his prediction is then translated for the awaiting public. The event was popularized in the 1993 movie of the same name, starring Bill Murray.
Sadly, it appears that global warming may soon add Phil to the ranks of the unemployed. With the warming of 4-8ºC (7-14ºF) predicted over North America by the end of this century if we continue to increase greenhouse gas concentrations at current rates, the answer will become simple. Spring will come early every year. While this may seem like a pleasant outcome of climate change, it could in fact lead to serious problems for plants, animals, and entire ecosystems. Living things have adapted to the timing of the seasons over many thousands of years. Here, we are changing the timing of the seasons on timescales of decades. Plants and animals just don’t adapt well to changes on such short timescales. More »
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