]The Arctic could potentially alter the Earth's climate by becoming a possible source of global atmospheric carbon dioxide. The arctic now traps or absorbs up to 25 percent of this gas but climate change could alter that amount.

The Arctic has been a carbon sink since the end of the last Ice Age, which has recently accounted for between zero and 25 percent, or up to about 800 million metric tons, of the global carbon sink. On average, the Arctic accounts for 10–15 percent of the Earth's carbon sink. But the rapid rate of climate change in the Arctic–about twice that of lower latitudes–could eliminate the sink and instead, possibly make the Arctic a source of carbon dioxide.

Carbon generally enters the oceans and land masses of the Arctic from the atmosphere and largely accumulates in permafrost, the frozen layer of soil underneath the land's surface. Unlike active soils, permafrost does not decompose its carbon; thus, the carbon becomes trapped in the frozen soil. Cold conditions at the surface have also slowed the rate of organic matter decomposition, allowing Arctic carbon accumulation to exceed its release.

But recent warming trends could change this balance. Warmer temperatures can accelerate the rate of surface organic matter decomposition, releasing more carbon dioxide into the atmosphere. Of greater concern is that the permafrost has begun to thaw, exposing previously frozen soil to decomposition and erosion.These changes could reverse the historical role of the Arctic as a sink for carbon dioxide. In the short term, warming temperatures could release more Arctic carbon to the atmosphere, and with permafrost thawing, there will be more available carbon to release.

On the scale of a few decades, the thawing permafrost could also result in a more waterlogged Arctic, a situation that could encourage the activity of methane-producing organisms. Currently, the Arctic is a substantial source of methane to the atmosphere: as much as 50 million metric tons of methane are released per year, in comparison to the 400 million metric tons of carbon dioxide the Arctic stores yearly. But methane is a very potent greenhouse gas – about 23 times more effective at trapping heat than carbon dioxide on a 100-year time scale. If the release of Arctic methane accelerates, global warming could increase at much faster rates. Scientists don't understand methane very well, and its releases to the atmosphere are more episodic than the exchanges of carbon dioxide with the atmosphere. It is important to pay attention to methane dynamics because of methane's substantial potential to accelerate global warming.

But uncertainties still abound about the response of the Arctic system to climate change. For example, global warming may produce longer growing seasons that promote plant photosynthesis, which removes carbon dioxide from the atmosphere. Also, the expansion of shrubs in tundra and the movement of tree line northward could sequester more carbon in vegetation. However, increasingly dry conditions may counteract and overcome these effects. Similarly, dry conditions can lead to increased fire prevalence, releasing even more carbon.

1. Based on the information in the passage, which of the following would not be likely to increase the amount of carbon dioxide in the Earth’s atmosphere?

A) An increased rate of decomposition of surface organic matter in the Arctic
B) Thawing of the Arctic permafrost
C) An increase in the rate of release of Arctic methane
D) Longer growing seasons in the Arctic
E) An increased number of fires in the Arctic



2. Based on the information in the passage, how likely is it that the Arctic will become a significant source of global atmospheric carbon dioxide?

A) Certain
B) Nearly certain
C) Very likely
D) Fairly likely
E) Unlikely



3. Which statements would the author be likely to agree with? Select all that apply

A) Scientists do not fully understand the dynamic interactions that lead to climate change.
B) Most of the carbon stored in the Arctic is in permafrost.
C) As more shrubs grow in the tundra as a result of warming trends, it is likely that more carbon dioxide will be released into the atmosphere, resulting in further warming.



4. Identify the sentence in which the author speculates that possible factors offsetting the increase in atmospheric carbon dioxide in the Arctic will be outweighed by other factors.