The climate is a complicated system which can take a long time to react to new inputs and we are all dependent on its vagueries so we can't really do experiments on it in order to understand how it works, and to fine tune our models of how it will behave in the future.
This means the only way we can study its behaviour is to look into what was going on in the past. The North Greenland Ice core Project is giving us our best resolution view of the climate for the last 15 years so far. This has involved drilling an ice core in northern Greenland and then putting the core into an automated machine at the drilling site. This machine then slowly melts the ice a few months worth at a time, and measures the composition of this ice.
This is useful because water containing oxygen-18 will freeze before the lighter oxygen-16 so the warmer the ice crystals in snow form the more oxygen-18 there will be, and water containing hydrogen-2, deuterium, will evaporate at a higher temperature than hydrogen 1 so the more deuterium in the ice the hotter the water that it evaporated from was.
Previous ice cores had shown that the temperature in Greenland increased by 10C in 50 years, this new data not only confirms this but shows that the ice cover on the Atlantic probably retreated from Portugal to Iceland in just 1-3 years. Also a few years before this melting the amount of very fine dust from Chinese deserts suddenly dropped indicating that the deserts suddenly became wetter.