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The four forecasts are obtained for different assumptions about winds and biology:

Upper left: drift with current only
Upper right: drift with current plus winds
Lower left: drift with current only, subject to growth and loss
Lower right: drift with current plus winds, subject to growth and loss

We estimate growth and loss (mortality) along each particle trajectory, according to surface temperature. Sargassum grows fastest at 26-27oC, but increasingly dies off at temperatures above 29oC. Through recent laboratory and mesocosm experiments, we now know more about the dependence of growth on temperature, in particular that the ‘new’ sargassum grows well at higher temperatures than previously thought, so these results may need updating.

Notice how variable the forecasts are, depending on the assumptions about winds and growth/loss. Notice also that each forecast is based on a number of ‘ensemble members’ that we average together (the thick purple line), accounting for considerable uncertainty in wind, currents and temperatures, for any particular year – a bit like the weather forecast.