Some ecological changes have important ecological consequences. Three well-documented global changes are increasing concentrations of carbon dioxide in the atmosphere, alterations in the biogeochemistry of the global nitrogen cycle, and ongoing land use or land cover change. Human activity, now primarily fossil fuel combustion, has increased carbon dioxide concentrations in the Earth's atmosphere.This is likely to have climatic consequences and direct effects on all of the planet's terrestrial ecosystems.
The global nitrogen cycle has been altered by human activity so much that more nitrogen is fixed annually by humanity than by nature. this added nitrogen alters the chemistry of the atmosphere and aquatic ecosystems, and has a substantial impact on biological diversity, especially in developed countries and in Asia, where vehicle emissions and industrial agriculture are highest.
Finally, human land use or land cover change has transformed half of the Earth's ice-free surface. This, in and of itself, probably represents the most important component of global change now and in the future, with profound implications for biological diversity around the world.
These three and other equally critical components of global environmental change are the primary causes of anticipated changes in climate, and of ongoing losses of biological diversity. They are caused, in turn, by the extraordinary growth in size and resource use of the human population.
What does "This" refer to?