Hi, I'm Bryan Yeaton and this is The Weather Notebook.

Over the past 150 years, our use of fossil fuels has more than doubled the CO2 in the atmosphere. But as Weather Notebook correspondent Allan Coukell reports, scientists are trying to determine the impact of CO2 on forests.

COUKELL: The leaves of plants use photosynthesis to take carbon (from carbon dioxide) out of the air. That's how they grow. But scientists studying the growth of forests are finding that increasing the amount of carbon dioxide in the air won't necessarily increase the growth of forests.

David Whitehead is a scientist from Landcare Research in New Zealand. He says it is a shortage of other nutrients that may limit the growth of forests where he's working.

WHITEHEAD: The work we've done on elevated CO2 does suggest that we're not going to see massive effects in terms of growth rates of our forests. The general view is that trees are not going to respond dramatically because there is always something else which is limiting the growth of trees, and in so many areas it's either nitrogen or phosphorus. And if you can't provide extra nitrogen, then however much carbon dioxide you've got you cannot increase growth.

COUKELL: One possible response to climate change is to rely on forests to soak up carbon dioxide from the atmosphere. So, it's important to understand how forests respond to changing conditions.

That's correspondent Allan Coukell of Auckland, New Zealand. The Weather Notebook is a production of the Mount Washington Observatory and underwritten by the National Science Foundation. Click on our website, mountwashington.org, for more on the atmosphere.