Graphic: 2010 Drought in the Amazon Forest @NASA @climate
The measurement of vegetation over tropical forests can be tricky. The vegetation index is made by combining the visible and infrared red light the plants reflect back to space. The dense, lush foilage in tropical forests absorbs most visible light, so little visible light makes it back to the satellite, making it more difficult to calculate the vegetation index than in other ecosystems. At the same time, smoke reflects visible light. When there is smoke in the atmosphere during the dry season, the extra reflected light can make it appear that the forest is less healthy than it is. During drought years, smoke can be more of a problem, since fire is more common in the dry conditions and the Amazon can be cloaked in near perpetual smoke.