Global Warming: Real or not, is it good or bad?

Cooling. Though the time at which the Eemian interglacial ended is subject to some uncertainty (it was probably around 110,000 years ago), what does seem evident from the sediment records that cross this boundary is that it was a relatively sudden event and not a gradual slide into colder conditions taking many thousands of years. The recent high-resolution Atlantic sediment record of Adkins et al (1997) suggests that the move from interglacial to much colder-than-present glacial conditions occurred over a period of less than 400 years (with the limitations on the resolution of the sediment record leaving open the possibility that the change was in fact very much more rapid than this).

Following this initial cooling event, conditions often changed in sudden leaps and bounds followed by several thousand years of relatively stable climate or even a temporary reversal to warmth, but overall there was a decline. Northern forest zones retreated and fragmented as the summers and winters grew colder. Large ice sheets began to grow in the northern latitudes when the snow that fell in winter failed to melt, and instead piled up from one year to the next until it reached thousands of metres in thickness.

As the cold grew more severe, the Earth's climate also became drier because the global 'weather machine' that evaporates water from the oceans and drops it on the land operates less effectively at colder temperatures and when the polar sea ice is extensive. Even in areas that were not directly affected by the ice sheets, aridity began to cause forests to die and to give way to dry grassland, which requires less water to survive. Eventually, much of the grassland retreated to give way to deserts and semi-deserts, as global conditions reached a cold, dry low point around 70,000 years ago (this is called the Lower Pleniglacial). By this time, most of northern Europe and Canada were covered by thick ice sheets.

When the sun gets dimmer, the Earth gets cooler; when the sun gets brighter, the Earth gets hotter. So important is the sun in climate change that half of the 1.5° F temperature increase since 1850 is directly attributable to changes in the sun. According to NASA scientists David Lind and Judith Lean, only one-quarter of a degree can be ascribed to other causes, such as greenhouse gases, through which human activities can theoretically exert some influence.

The correlation between major changes in the Earth's temperature and changes in solar radiance is quite compelling. A perfect example is the Little Ice Age that lasted from 1650 to 1850. Temperatures in this era fell to as much as 2° F below today's temperature, causing the glaciers to advance, the canals in Venice to freeze and major crop failures. Interestingly, this dramatic cooling happened in a period when the sun's radiance had fallen to exceptionally low levels. Between 1645 and 1715, the sun was in a stage that scientists refer to as the Maunder Minimum. In this minimum, the sun has few sunspots and low magnetism which automatically indicates a lower radiance level. When the sun began to emerge from the minimum, radiance increased and by 1850 the temperature had warmed up enough for the Little Ice Age to end.

The Maunder Minimum is not an isolated event: it is a cyclical phenomenon that typically appears for 70 years following 200-300 years of warming. With only a few exceptions, whenever there is a solar minimum, the Earth gets colder. For example, Europe in the 13th and 15th Centuries experienced significantly lower temperatures and in both cases the cold spells coincided with a minimum. Similar correlations were found in the 9th Century and again in the 7th Century. Since 8700 B.C., there have been at least ten major cold periods similar to the Little Ice Age. Nine of those ten cold spells coincided with Maunder Minima.