[aerohead]

We have millions of years of climate data via proxy data.Tree ring dendrochronolgy,geologic stratification,stalagtites,corals,ice -cores,sediment- cores,foraminifera,isotopic assays of carbon-14,berylium-10,oxygen-18,etc.,have allowed researchers to reconstruct the past.Air bubbles trapped in the ice sequester carbon dioxide,methane,etc.,which allow paleo-atmospheric reconstructions.Sea-levels can be ascertained.Isotopes tell us about cocmic particle activity,hence solar wind,hence solar minimum-maximums/galactic/intersteller cosmic particle bombardment.We can tell if carbon came from oil,natural gas,forest fires,peat bogs,or coal.We can witness volcanic activity,the parasol effect,acid rain.Temperatures can be derived from greenhouse gases trapped in the ice.
There are hundreds of years of ships logs which contain soundings (mark twain),temperatures at depth,surface water temperatures.We have declassified,formerly,top-secret ship log data from US,British,and Soviet naval vessels/submarines, from the Cold War era.Balloon data since before rockets.Rocket-derived data.
In 1978 we began satellite remote-sensing.We have limited terrestrial weather observations from countries and colonial holdings.Thermometers and barometers have been around a long time.We've had solar observations continuously since 1611.
Volcanic eruptions are written in the ice.
We know when changes to ice bridges, glacier-melt inland seas,and major tectonic /geographic features happened.The three Milankovich Earth orbital perturbations are accounted for,to an accuracy of fractions of an arc-second.Magnetospheric polar reversals are recorded in the rocks.Atmospheric photochemistry is well understood.Computers get faster.Numerical models get more sophisticated,with ever higher resolution,and results arrive sooner.
We know enough to model the climate to above a 90% accuracy,going backwards and forwards.