Hot and cold Earth through time

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By Benjamin J. W. Mills, Science. 

Excerpt: What was Earth’s temperature tens to hundreds of millions of years ago? The planet has gone through different periods, some with extensive polar ice caps and others being completely ice-free. ...there have been major disagreements about whether there has been an overall decline in Earth’s temperature over time. ...Judd et al. [in A 485-million-year history of Earth’s surface temperature] report a new reconstruction of Earth’s temperature over the last 485 million years by combining climate models with geological data. ...Earlier studies using only tropical oxygen isotope ratios predicted a long-term decline in temperature over the last 500 million years, proposing that earlier greenhouse periods were warmer than more recent ones.... The new reconstruction of Judd et al. disagrees and instead predicts that greenhouse periods had similar temperature ranges. ...Direct comparison of a possible future greenhouse climate to the past ones remains difficult because those warm periods were established gradually over millions of years. However, they are the only evidence available for what greenhouse climates look like, and they are vital for testing the accuracy of climate models.... 

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