Celestial climate driver: a perspective from four billion years of the carbon cycle.

SUMMARY

The standard explanation for vagaries of our climate, championed by the IPCC (Intergovernmental Panel on Climate Change), is that greenhouse gases, particularly carbon dioxide, are its principal driver. Recently, an alternative model that the sun is the principal driver was revived by a host of empirical observations. Neither atmospheric carbon dioxide nor solar variability can alone explain the magnitude of the observed temperature increase over the last century of about 0.6[degrees]C. Therefore, an amplifier is required. In the general climate models (GCM), the bulk of the calculated temperature increase is attributed to "positive water vapour feedback". In the sun-driven alternative, it may be the cosmic ray flux (CRF), energetic particles that hit the atmosphere, potentially generating cloud condensation nuclei (CCN). Clouds then cool, act as a mirror and reflect the solar energy back into space. The intensity of CRF reaching the earth depends on the intensity of the solar (and terrestrial) magnetic field that acts as a shield against cosmic rays, and it is this shield that is, in mm, modulated by solar activity.

Cosmic rays, in addition to CCN, also generate the so-called cosmogenic nuclides, such as beryllium-10, carbon-14 and chlorine-36. These can serve as indirect proxies for solar activity and can be measured e.g., in ancient sediments, trees, and shells. Other proxies, such as oxygen and hydrogen isotopes can reflect past temperatures, carbon isotopes levels of carbon dioxide, boron isotopes the acidity of ancient oceans, etc. Comparison of temperature records from geological and instrumental archives with the trends for these proxies may enable us to decide which one of the two alternatives was, and potentially is, primarily responsible for climate variability. This, in turn, should enable us to devise appropriate countermeasures for amelioration of human impact on air quality and climate.

INTRODUCTION

Carbon dioxide, generally believed to be the most important greenhouse gas and climate modifier, is todav the focus of a heated political and scientific debate that has polarized scientists, policy makers, and the public. One side maintains that C[O.sub.2] is the principal driver of climate, with the Intergovernmental Panel on Climate Change (IPCC, 2001) projecting a global mean temperature rise from 1.5 to 5.8[degrees]C by the year 2100. The other side (e.g., Douglass et al., 2004) claims that the role of anthropogenic C[O.sub.2] on climate has not been proven, and that there is therefore no need for emissions quotas such as those mandated by the Kvoto Protocol.

As is usually the case with contentious matters, the reality likely lies somewhere in between. So why is this issue so polarizing? First, past, natural, variations in the carbon cycle and climate are poorly understood. These variations must be taken into account as a baseline for any superimposed human impact. Second, the climate models are, at best, only an approximation of reality. Since I am a geologist and not a modeller, I will deal mostly with the empirical record of climate and the carbon cycle, contemplating them at time scales ranging from billions of years to the human life span (Fig. 1). This perspective is essential, because events on progressively shorter time scales are embedded in, and constrained by, the evolution of the background on longer time scales.

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