Preserving Life on Planet Earth

This is the last of four blog posts by PSR intern Frederik Lichtenberg and Environment & Health director Barbara Gottlieb, reflecting on a recent article by NASA climatologist James Hansen.

In their article, “Perceptions of Climate Change: The New Climate Dice,” Hansen and his coauthors ask an important question:  If we want to identify changes in the world’s climate, what do we compare current temperatures against?  What is the right base period to examine?  The question matters.  After all, if we adopt as our norm a time period where changes in climate are already in place, we will fail to recognize just how large recent changes have been. 

After 10,000 years, big changes

Hansen argues that when choosing a base period, it is sensible to pick one that represents the climate conditions that have existed for most of humanity’s life on Earth:   the past ten thousand years.  Over that period, known as the Holocene period, the climate remained relatively stable.  In contrast, the planet has warmed considerably in just the past thirty years.  This is reflected in the fact that the major ice sheets in both the northern and southern hemispheres are melting much more rapidly, and that sea level is rising more rapidly, than at any previous point in the Holocene.

Baseline:  Comparing to what?

Hansen picks as his baseline a time period that precedes those recent, rapid changes.  His choice:  1951-1980, a period which represents the climate that life on Earth has adapted to, and for which we have reliable global data.

Other researchers have chosen as their base period the past three decades, 1981-2010. While it’s reasonable to look at a recent multi-decade period, the problem with this choice, as Hansen et al explain, is that those years had already experienced an increase in average temperatures of about 0.5°C.   Picking a warmer period as the basis for defining “normal” serves to disguise the fact that today’s normal is significantly higher than the norm just two generations ago. 

In fact, the changes witnessed in the years 1981-2010 already indicate the rapid changes in climate that humankind will likely have to adjust to in the very near future.

Changes in the water cycle

The changes arising from increasing global temperatures will have their biggest impacts on the hydrological cycle. Hot and dry places are expected to become even drier. At the same time, unusually heavy rainfall and floods will also become more common.  Since warm air masses can store more water than cool ones, the total amount of water vapor held by the atmosphere will soar as temperatures rise, leading to more precipitation falling in extreme events. Formerly ‘100-year’ and ‘500-year’ storms and floods will lose their historic character as their frequency surges in the future.

Current data already show increases of such events over many areas of the Northern Hemisphere as well as in the tropics. Studies link this trend to human-induced global warming.

Effects on insects, disease vectors

Hansen and his co-authors stress that climatic changes are manifested not only by droughts, floods and other extreme events. There are numerous subtler indicators of climate change, including effects on animals, birds, insects and plants.  These include the epidemic of pine bark beetles destroying forests in Canada and the U.S. Rockies, and the geographical spread of such diseases as West Nile virus and chikungunya.

While certain species, including those that transmit disease, spread with global warming, other species, especially those habituated in the tropics, are likely to go extinct.  If indeed the mean global temperature increases by about 3°C, it is estimated that 21 to 52 percent of all species on our planet will vanish.

This is extraordinary, since many animal species migrate long distances in order to stay in their preferred and needed climatic zone. However, for many species it will be impossible to survive such a harsh and rapid increase in temperature.

Survival:  Still possible

It is still possible to avoid such an alarming scenario by taking the appropriate measures to fight global warming. Dramatically increasing energy efficiency and conservation, replacing fossil fuels with non-polluting forms of energy, and slashing carbon emissions where fossil fuels are burned would all move the world in the right direction.

That would allow us to limit global warming to less than one degree Celsius, preserving life as we have come to know it on planet Earth. 

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