 Uncertainty arises from a number of different sources when we're talking about projections of future climate change. First of all, with each individual climate model we use to produce these projections, each model gives a little bit different result. And so you get a spread of results across a number of different climate models. So you can quantify the uncertainty just based on the differences among the models. That's one source of uncertainty when you're talking about a possible future climate change. Another source of uncertainty is there's a lot of different possible scenarios of what could happen over the next hundred years or so in terms of what the emissions of greenhouse gases are going to be like. This has to do with what we think future population growth may be like, what different types of energy sources we'll be using over the next hundred years. And so we get a spread of possible outcomes say at the year 2100 just based on possible different scenarios of all these factors that affect greenhouse gas emissions. The other type of uncertainty and the type of uncertainty that I study primarily is how do we represent the climate system in a climate model. And as I indicated earlier, we have several major challenges in that area. For example, how do we represent clouds and how do we treat complicated atmospheric aerosols, the particles of scattered light. Those are two major challenges for us and at the moment two major sources of uncertainty. One thing that we have not included is the changes to the land surface that come from human land use, for example. Is this a key uncertainty for the future? We do not quite know exactly, we think it's not very large, the contribution is not very large. However, we haven't included it yet. So that is an uncertainty that still is there. If the land surface will look very different a hundred years from now, we would have to adjust our climate predictions in the sense with the climate model output. There's particularly more uncertainty about exactly what will happen on regional scales compared to global scales. And the regional scales are the ones we're most interested in because that's where people have to make decisions from regional to local scales. Even with these uncertainties, it's possible to make decisions now about how we will adapt to the future. And that's very important to realize that we don't need all the details. To make some decisions now.