Flowering plants leave Earth cold.

Flowering plants leave Earth cold

The dinosaurs roamed a world much hotter than the one we inhabit. Since the Cretaceous period, which ended 65 million years ago, the Earth's surface has cooled substantially, perhaps by an average 5[deg.] to 10[deg.]C. While scientists usually invoke platetectonic activity to explain this temperature drop, a climate researchers from New York University proposes that a certain class of plant may have played a large role.

The end of the Cretaceous marks the transition from the age of reptiles to the age of the mammals, but the plant world was playing out its own drama back then as well. At that time, the angiosperms or flowering plants -- a group that includes grasses and deciduous trees -- were starting to compete seriously with the more established type of land plant called gymnosperms, which includes coniferous trees. In the February GEOLOGY, Tyler Volk suggests the rise of the angiosperms could have cooled the Earth's surface by lowering the amount of carbon dioxide in the atmosphere and thereby weakening Earth's greenhouse effect.

Before humans entered the picture and started burning fossil fuels, the amount of carbon dioxide in the atmosphere represented a balance between two general forces: volcanic-tectonic processes that put carbon dioxide into the air, and chemical weathering of rocks that pulls carbon dioxide out of the air. Scientists traditionally have attributed the post-Cretaceous drop in atmospheric carbon dioxide to the slowing down of the source of the gas -- namely tectonics.

However, Volk points out, angiosperm-deciduous ecosystems weather rocks much faster than conifer-evergreen systems, for reasons scientists don't understand. By constructing a model, he shows that the spread of angiosperms since the late Cretaceous must have increased the global weathering rates of rocks, which would lower atmospheric carbon dioxide and cool the world. In the past, scientists dealing with the carbon cycle have assumed that biological influences on weathering have not changed over the eons. Volk's theory joins a growing movement by scientists to examine how biological changes can affect the climate.