Climate change and science: a response to Benny Peiser, William Connolley Ray Bradley Michael Mann Gavin Schmidt

Mon, 16 May 2005 23:00:00 +0000

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In order to choose policy options wisely, decision-makers require the most accurate assessment of the relevant scientific information without having followed every discussion in the technical literature. To that end, scientists have a public role in placing new results in context, distilling the current state of knowledge, and pointing out where continuing uncertainties and problems lie.

We comment here on scientific content in the submission by Benny Peiser in openDemocracy’s climate change debate. Any criticism of his scientific points is however independent of our opinions concerning his preferred policy options.

Benny Peiser raises two principal issues: continuing uncertainty in the monitoring of the planet’s albedo (or reflectivity), and the role of solar-related variability in forcing the Earth’s climate. We deal with each point in turn.

Albedo (reflectivity) is an important quantity for the energy balance of the planet. It is affected by the amount of cloud, snow and ice cover, vegetation and the aerosol content of the atmosphere. Measuring it directly, however, is very difficult: it requires instruments to calculate radiation over a huge range of frequencies, and since it is so locally variable due to cloud variability, there are significant sampling problems.

Estimates of the planet’s albedo are around 0.3+/-0.01, but the exact number and its potential variation over time are highly uncertain. A recent paper (Wielicki et al, Nature 2005) has produced satellite-based results completely at odds with a previous study that used Earthshine measurements from the moon (Palle et al, Nature 2004). Clearly then, changes to the planet’s albedo are currently unclear.

However, other integrated measures of the Earth’s net energy balance are available, in particular the changes in ocean heat content. The increase of energy in the ocean is linked directly to the energy balance at the top of the atmosphere because the oceans are by far the biggest repository of heat in the climate system.

The oceans have been heating up at about 0.6 W/m² over the last ten years (Willis et al, Nature 2004) and at a slightly lower rate since 1950 (Levitus et al, Nature 2000). This can only have occurred with a net planetary imbalance of radiation (i.e. the Earth is absorbing more energy than it is giving out). Thus, regardless of changes in the albedo, the absorbed solar radiation has been less than the outgoing long-wave radiation over this period, and in particular over the last decade.

In addition, climate models driven by our current best guesses for changes in greenhouse gases, aerosols, and solar forcing predict a planetary energy imbalance very close to that deduced from the ocean heat content results (Hansen et al, Science 2005). Improvements to the albedo record will surely help further refine climate models, and particular the net cloud feedbacks, but the current uncertainties do not undermine the dominant role for greenhouse gases in altering the present energy balance of the planet.

Solar variability has long been a candidate for causing climate variations on Earth. Direct measurements of the total solar irradiance (TSI) have only been available from satellites since 1979 and even these are subject to considerable uncertainties. Before then, long-term estimates have relied on highly uncertain correlations with the sunspot record and cosmogenic isotopes measured in ice cores. Measured TSI variations on their own (on the order of ~0.1%) are too small to have had a large role in recent climate variability; however, there are indirect effects (such as stratospheric ozone feedbacks) which can increase the impact of solar variability.

These effects are included in current climate models but over the last fifty years or so do not contribute significantly to the ongoing warming trends. Another postulated effect is the impact of cosmic rays on cloud nucleation in the atmosphere. These rays are modulated by solar magnetic activity and thus also vary in phase with the sunspot cycle. However, this mechanism remains highly speculative and has not been clearly demonstrated to occur in the real world.

Regardless of the specifics, though, it should be pointed out that all solar activity indices (such as sunspots, helio-magnetic records, cosmic rays) generally peaked in the late 1950s and have declined or held steady since then. Thus solar activity is unlikely to be contributing to current (post-1970) increases in surface temperatures or the planetary energy imbalance.

In an additional aside, Benny Peiser quotes Jan Vezier’s statement that "greenhouse gases [act] only as potential amplifiers". While this might seem to contradict the consensus position, it is in fact obviously true for changes to greenhouse gases prior to the industrial era. It is precisely that amplifying effect that models suggest will lead to approximately 2-4 degrees Centigrade warming if C02 doubles. In the current situation where humans have significantly increased greenhouse gases above background levels, this amplification is happening in the absence of large external drivers (whether these are celestial, volcanic, orbital or tectonic).

We therefore conclude that none of these issues raised by Benny Peiser have much, if any, implication for the consensus on the principal climate debate issue: the impact of anthropogenic greenhouse gases.

This article appears as part of openDemocracy‘s online debate on the politics of climate change. The debate was developed in partnership with the British Council as part of their ZeroCarbonCity initiative – a two year global campaign to raise awareness and stimulate debate around the challenges of climate change.

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Climate change and science: a response to Benny Peiser, William Connolley Ray Bradley Michael Mann Gavin Schmidt