https://quadrant.org.au/magazine/2014/01-02/fundamental-uncertainties-climate-change/
The World Meteorological Organisation of the United Nations took its first steps towards establishing the World Climate Program in the early 1970s. Among other things it held a conference in Stockholm to define the main scientific problems to be solved before reliable climate forecasting could be possible. The conference defined quite a number, but focused on just two. The first concerned an inability to simulate the amount and character of clouds in the atmosphere. Clouds are important because they govern the balance between solar heating and infrared cooling of the planet, and thereby are a control of Earth’s temperature. The second concerned an inability to forecast the behaviour of oceans. Oceans are important because they are the main reservoirs of heat in the climate system. They have internal, more-or-less random, fluctuations on all sorts of time-scales ranging from years through to centuries. These fluctuations cause changes in ocean surface temperature that in turn affect Earth’s overall climate.
The situation hasn’t changed much in the decades since. Many of the problems of simulating the behaviour of clouds and oceans are still there (along with lots of other problems of lesser moment) and for many of the same reasons. Perhaps the most significant is that climate models must do their calculations at each point of an imaginary grid of points spread evenly around the world at various heights in the atmosphere and depths in the ocean. The calculations are done every hour or so of model time as the model steps forward into its theoretical future. Problems arise because practical constraints on the size of computers ensure that the horizontal distance between model grid-points may be as much as a degree or two of latitude or longitude—that is to say, a distance of many tens of kilometres.