Global warming house

Carbon dioxide, the greenhouse gas linked to global warming, is accumulating in the Earth’s atmosphere at an increasing rate, according to a new study released by the US government’s National Oceanic and Atmospheric Administration. The research has renewed concern that the ability of the environment to absorb the gas may be waning. The NOAA study said the average atmospheric concentration of carbon dioxide in 2005 reached 381 parts per million, up from 2.6 ppm since 2004. The annual rate of increase, which has been recorded since the 1950s, now exceeds 2 ppm for three of the past four years. This is an unprecedented increase; 50 years ago, the annual increase was less than 1 ppm.

The extra CO2 is produced by the burning of fossil fuels, currently emitting approximately 7 billion tons of carbon per year, and roughly half is absorbed by vegetation and the oceans. Researchers believe the yearly fluctuations in CO2 build-up are caused largely by nature's variable ability to absorb the emissions. The atmospheric concentration of carbon dioxide is now higher than experienced on Earth for at least the last 400,000 years, and the rise is expected to continue. Over the past two decades, only half of the CO2 released by human activities such as fossil fuel burning, the so-called “anthropogenic CO2,” is still in the atmosphere; about 30% has been taken up by the ocean, and 20% by the terrestrial biosphere.

This new finding follows reports that 2005 was probably the warmest year on record, with temperatures slightly higher than the previous peak in 1998. Also, scientists at the US National Snow and Ice Data Center, in Boulder, Colorado, reported that Arctic sea ice did not reform fully in the winter of 2005 after record rates of melting during the summer.

Until recently the largest increases in concentrations of CO2 always occurred during El Niño years, when tropical vegetation grows more slowly due to lack of rain and fires occur in dried-out rainforests. The greatest recorded increase of 2.7 ppm occurred in the El Niño year of 1998. However, scientists are alarmed by the fact that none of the past three years of near-record increases have coincided with an El Niño event.

According to Peter Cox, a scientist at the Center for Ecology and Hydrology in Dorset, UK, who studies the interaction between plants and the atmosphere, the recent surge in CO2 levels “may be the first evidence of a feedback from the carbon cycle, in which plants under heat stress from global warming start to absorb less carbon dioxide”.