Tiny microfossils called Foraminifera form calcium carbonate shells that record the δ18O composition of the seawater in which they grew. These microfossils are found in sea bed sediment cores, and a lot of information from these oxygen isotope records can be extracted. ‘Reconstructing the past and weathering the future’ looks at the methodology used in palaeoclimate studies and explains gain and phase modelling and Milankovitch orbital cycles. Similar isotope temperature records have been constructed from polar ice cores. Atmospheric CO2 composition can be reconstructed from the amount of CO2 dissolved in the ice. A new sub-discipline of clumped isotope geochemistry—‘isotomics’—will have applications far beyond carbonate palaeothermometry.