Abstract

Recent analyses of Amazon runoff and gridded precipitation data suggest an intensification of the hydrological cycle over the past few decades in the following sense: wet season precipitation and peak river runoff (since approximate to 1980) as well as annual mean precipitation (since approximate to 1990) have increased, while dry season precipitation and minimum runoff have slightly decreased. There has also been an increase in the frequency of anomalously severe floods and droughts. To provide context for the special issue on Amazonia and its forests in a warming climate we expand here on these analyses. The contrasting recent changes in wet and dry season precipitation have continued and are generally consistent with changes in catchment-level peak and minimum river runoff as well as a positive trend of water vapor inflow into the basin. Consistent with the river records, the increased vapor inflow is concentrated to the wet season. Temperature has been rising by 0.7 degrees C since 1980 with more pronounced warming during dry months. Suggestions for the cause of the observed changes of the hydrological cycle come from patterns in tropical sea surface temperatures (SSTs). Tropical and North Atlantic SSTs have increased rapidly and steadily since 1990, while Pacific SSTs have shifted during the 1990s from a positive Pacific Decadal Oscillation (PDO) phase with warm eastern Pacific temperatures to a negative phase with cold eastern Pacific temperatures. These SST conditions have been shown to be associated with an increase in precipitation over most of the Amazon except the south and southwest. If ongoing changes continue, we expect forests to continue to thrive in those regions where there is an increase in precipitation with the exception of floodplain forests. An increase in flood pulse height and duration could lead to increased mortality at higher levels of the floodplain and, over the long term, to a lateral shift of the zonally stratified floodplain forest communities. Negative effects on forests are mainly expected in the southwest and south, which have become slightly drier and hotter, consistent with tree mortality trends observed at the RAINFOR Amazon forest plot network established in the early 1980s consisting of approximately 150 regularly censused 1ha plots in intact forests located across the whole basin.