Lawns May Contribute to Global Warming

My uncle, who has a Ph.D in chemistry, and while not necessarily a global warming specialist, is skeptical of the idea. I think global warming is happening, but it's not necessarily caused by us. Back at the end of the dark ages, temperatures rose and more land became fertile, partially why Europe was able to lift itself out of the dark ages. Temperatures rise and fall, and it's not necessarily dependent on us.