evidence: These clues to past climate are scattered across the Earth, from the layers of vast ice sheets and sediments at the bottom of lakes to rings of tree growth and towering stalagmites in caves. (See later section for more on the different sources of proxy data.) [...] For example, explains Prof Dan Lunt, professor of climate science at the University of Bristol, "50m years ago, CO2 concentrations were greater than today and the planet was substantially warmer. Proxies for CO2 allow us to quantify the former and proxies for temperature allow us to estimate the latter". Scientists can use this information to estimate climate sensitivity. [...] Proxy data can also help determine changes to sea level, ice cover and vegetation during past warm periods. One example is the previous interglacial period around 125,000 years ago – called the Eemian that was likely as warm or warmer than today – which can provide evidence on how all of these factors may change as the world warms.