The Cretaceous has long been known as a very warm time of Earth’s history. Lyell (1837) pointed out the presence of Cretaceous chalk as far north as Denmark and Sweden as indicators of warm northerly oceans. This is now more reliably hinted at by the presence of vegetation at polar latitudes (Creber & Chaloner, 1985) – trees can’t grow on ice, so if they’re present on a polar landmass, it must have not been covered by an ice sheet. The Late Cretaceous is indeed the warmest period of Earth’s history from the past 144 Ma.
For the early Cretaceous, one can take the climate of the well-studied Wealden Beds of southern England as exemplary of the general global climate. There was a seasonal climate alternating between fairly warm average temperatures of 25°C and going down to cooler 10°C (Allen et al., 1998), with temperatures going as high as 40°C and as low as 4°C, respectively (Haywood et al., 2004). Elsewhere on the globe, there were some arid zones and some humid, tropical zones, as well as glaciated poles. It wasn’t identical to today, but similar at a general level, just a bit warmer and wetter.
The mid-Cretaceous saw one of the more dramatic climate transitions in the history of the Earth (Hay, 2011) with temperatures becoming 6-14°C warmer (Barron, 1983). Increased volcanism led to higher levels of CO2 and caused the appearance of a greenhouse climate where even the poles experienced average temperatures between 13 and 20°C on ocean and land, respectively (Jenkyns et al., 2004). The Arctic was populated by crocodile relatives (Tarduno et al., 1998), and since modern crocs don’t live in freezing temperatures, it’s inferred that the Arctic must have been at least temperate.
The mid-Cretaceous transition enabled proper temperate forests to emerge where now there is only tundra or ice (Spicer et al., 1993). This expansion of forests then led to a positive feedback reinforcing the warming trend to form a supergreenhouse in the Late Cretaceous, since the forest cover lowered the albedo of the land and thus helped heat retention (Upchurch et al., 1998).For an example of how the climates of the time were, one can look at the Two Medicine Formation in the USA, where tree rings indicate the exact same kind of climate in the Western Interior of the USA as in East Africa today (Falcon-Lang, 2003).
In the Arctic, it was warm enough that there was only some sea ice in winter, but otherwise it was all ocean (Davies et al., 2009) – the same state we are expecting to have by the next 50 years (Holland et al., 2006). This is why the Cretaceous is a good palaeoclimate analog to modern global warming, especially for studying the effects of having no polar glaciation. It’s also an effective testing ground for the accuracy of climate models. It has long been a problem that climate models taking only CO2 levels into account underestimated the true magnitude of the warming, leading to the refining of climate models to include other factors, including circulation patterns, ocean temperatures, geography, topography, and other parameters to make the models more accurate. These lessons are then applied into modern climate modelling, leading to the very precise models used nowadays.
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