Ice core data from the past 600,000 years show that when CO2 is high, global temperatures are high. Although current CO2 levels are higher than they have ever been, temperatures have not caught up with greenhouse gas levels due to thermal inertia. Scientists say that it is only a matter of time before temperatures rise to match atmospheric CO2 levels. Even with thermal inertia, the hottest years of the last 1,000 years have been in the past two decades, and the numbers of temperature records that have recently been broken indicate that the trend is continuing. Nations vary greatly in the amount of greenhouse gas emissions that they add to the atmosphere, with the United States and China in the lead.

Without a doubt, the Earth's climate future will include the enormous impact that humans make on the atmosphere. Nearly all climate scientists agree that human influence will overshadow natural changes for at least a millennium, until Milankovitch and other natural cycles push the planet toward a new ice age.



Effects of Climate Change on the Atmosphere and Hydrosphere

No single event can be attributed unequivocally to global warming: not ice melting, not an increase in hurricane intensity, not the bleaching of coral reefs. It is the sum of all of these changes collectively that points very strongly to a world in which global warming is having an increasing effect. The most dramatic impacts being felt so far in the atmosphere and hydrosphere are the melting cryosphere, rising seas, and the rise in extreme weather events.

Many of the observations presented in this chapter and the next were described in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment. Much of what is presented in the report, and much of what is known about changes caused by warming tem--perature, comes from studies in the Northern Hemisphere because that is where the scientists are concentrated. Europeans, in particular, have been collecting information over decades and centuries that is useful today.

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