Climatic shifts have been a regular occurrence in the earth's history. Though highly reliable instrument
records have only been maintained for the past 300 years, naturally occurring geological, cryogenic and
stratigraphic evidences indicate dynamic shifts in the earth's surface climates over several thousand
years. These changes have been caused by various factors, such as variations in the earth's orbit around
the sun, tectonic movement, variations in solar output, eruption of volcanoes and meteorite impacts. The
most repetitive cause was change in concentrations of greenhouse gases in the atmosphere. All of these
factors caused a significant cooling or warming of the earth's atmosphere, and resulted in massive
planetary changes. The most well known of such phenomena are the infamous and intriguing ice ages.
Ice ages that occurred in the past provide ample evidence of changing climates and phenomenon
associated with them. Ice is excellent at preserving occurrences over millions of years, and studying ice
reveals lots of information about conditions and events in the earth's history, especially during ice-ages.
There have been four major ice ages in the earth's history, during which extensive ice cover was found on
earth. Ice ages occur over millions of years, and are punctuated by glacial periods during which colder
temperatures occur and glaciers advance, and inter-glacial periods, during which warmer climes prevail
and glaciers retreat.
The present ice age began 40 million years ago with the growth of an ice sheet in Antarctica. It intensified
during the late Pliocene, around 3 million years ago, with the spread of ice sheets in the Northern
Hemisphere. Since then, the world has seen cycles of glaciations with ice sheets advancing and retreating
on 40,000 and 100,000-year time scales. Glacial periods are also region specific, such as the Riss (180-
130,000 years ago) and Wurm (70,000-10,000 years ago), which occurred largely in the European Alps.
Their end signaled the start of the Holocene inter-glacial period (10,000 years ago present day), which is
continuing till date.
The most recently documented period of an ice age was the Little Ice Age (LIA). Starting in the 13th century in Europe, the LIA was a minor event of global cooling that lasted for nearly 2 centuries, and serves as an encore for the immediate future. The effects were primarily felt in Europe and the Northern Hemisphere, with Arctic ice packs and Greenland ice sheets advancing southwards & glaciers in the Swiss Alps engulfing entire villages.
The winters in Europe were particularly harsh, with entire populations succumbing to food shortages and cold related deaths. Glaciers covered several mountain ranges in tropical latitudes such as the Ethiopian highlands, and the famous 'snows of Kilimanjaro' advanced rapidly during this period. Established evidence points towards natural causes. The LIA was caused by heightened volcanic activity, emitting ash clouds and blocking solar radiation, causing worldwide cooling and triggering a small ice age.
However, it is virtually certain from the highly reliable Intergovernmental Panel on Climate Change (IPCC)
reports that present climatic shifts are human originated, and are being accelerated by human activities.
The current concentrations of carbon dioxide in the atmosphere, at 379 ppm, have far exceeded the
historic range of 230 300 ppm observed over the past 650,000 years. Cumulative greenhouse gas
concentrations have reached an unprecedented 430 ppm, and are rising at 2 ppm per year (greenhouse
gases are gases such as water vapour, carbon dioxide, methane and nitrous oxide that allow incoming solar
radiation to pass through the Earth's atmosphere, but prevent most of the outgoing infrared (heat)
radiation from the surface and lower atmosphere from escaping into outer space). If nothing is done to
control greenhouse gas emissions right away, we may witness the advent of an ice-age in our own lifetime.
|