Weather concerns an initial value problem: Given today's situation, what will tomorrow bring? Weather is chaotic; imperceptible differences in the initial state of the atmosphere lead to radically different conditions in a week or so. Climate is instead a boundary value problem — a statistical description of the mean state and variability of a system, not an individual path through phase space.
Note the words: A statistical description of the mean state and variability of the system. Every word is important. Climate is more than an average of weather. It also describes the extremes of weather, and the extent to which weather is likely to vary within those extremes.
When modeling climate and changes in the earth system, there are certain constraints, or limits, or boundaries. These constraints can include surface boundaries such as continental configurations, topography, bathymetry (topography of the sea floor), and vegetation distribution. On shorter time scales there are constraints relating to ice sheets, oceans and the atmosphere itself. Defining these constraints or boundaries is what scientists do when designing models of the earth system and climate.
This article is about two concepts - both involving the word "boundary", which need to be distinguished:
- The boundaries to a given climate (extremes of weather), and
- Boundary conditions, which are set when developing models of climate or the earth system as a whole. These can be any physical constraints.
Willis Eschenbach wonders about boundary conditions
Willis Eschenbach today is wondering what is meant by the boundary conditions problem for climate (archived here). At WUWT he wrote:
I’ve heard many times that whereas weather prediction is an “initial-value” problem, climate prediction is a “boundary problem”. I’ve often wondered about this, questions like “what is the boundary?”. I woke up today thinking that I didn’t have an adequately clear understanding of the difference between the two types of problems.
