Related by context. Frequent words. (Click for all words.) 69 CO2 concentrations 67 atmospheric CO2 66 atmospheric carbon dioxide 62 atmospheric concentrations 60 greenhouse gas concentrations 55 terrestrial ecosystems 54 Medieval Warm Period 54 ozone concentrations 53 greenhouse warming 53 degree centigrade 53 degrees Fahrenheit 53 N2O 53 nitrogen fixation 52 glaciation 52 carbon dioxide 52 CO2 52 ocean acidity 52 ° C 52 hydrological cycle 52 GPa 51 CO ² 51 mid latitudes 51 º C 51 #ppm [001] 51 ozone depletion 51 glacial melt 51 gigatonnes 51 anthropogenic 50 soils 50 ppm 50 acidification 50 Little Ice Age 50 species extinctions 50 temperatures 50 ˚ C 50 diatom 49 irradiance 49 carbon dioxide CO2 49 stratospheric ozone 49 interglacial 49 aerosol particles 49 nitrous oxide emissions 49 Supplementary Fig 49 temperature 49 sunspot activity 49 mass extinctions 49 centigrade 49 degrees centigrade 48 salinity 48 temperate zones 48 Carbon dioxide emissions 48 troposphere 48 degree Celsius 48 abundances 48 permafrost 47 volcanism 47 degrees Celsius 47 temperature gradient 47 Greenhouse gases 47 gigatons 47 denitrification 47 temperate regions 47 albedo 47 ° Celsius 47 ambient temperature 47 anthropogenic climate 47 latitudes 47 PETM 47 degree Fahrenheit 47 Carbon dioxide 47 radioactive decay 47 subtropical regions 47 biosphere 47 extinctions 47 atmospheric circulation 46 aridity 46 sulfate 46 anoxic 46 lake sediments 46 evapotranspiration 46 ^ sub 46 Sulfur dioxide 46 maxima 46 lithosphere 45 degrees Centigrade 45 greenhouse gases 45 water vapor 45 diatoms 45 fluxes 45 methane concentrations 45 multicellular organisms 45 decadal 45 pikas 45 Concentrations 45 X ray emission 45 photosynthetic 45 phytoplankton 45 climatic 45 ozone 45 oxygen isotopes 45 airborne pollutants