4. Case study - Climate extreme events in Amazonia: imminent threats to human security
The last seven years have featured severe droughts and floods in Amazonia, with some of these events characterized at the time as “once in a century”
seasonal extremes. These relatively recent extreme climatic events in the Amazon demonstrate the potential threat of such events to water security
for humans and for ecosystems. Droughts were experienced in 2005 and 2010 while severe floods occurred in 2009, 2011 and 2012 in various sectors
of the Amazon.
Various studies have shown that inter-annual variability of rainfall and consequently of rivers in the Amazon region is in part attributed to
variations in sea surface temperature SST in the tropical Pacific, manifested as the extremes of El Niño-Southern Oscillation ENSO, and in
the meridional SST gradient in the tropical Atlantic, or to a combination of both See reviews in Ronchail et al 2002, Zeng et al 2008, Yoon and Zeng
2010 and Marengo et al 2008, 2011c, d, 2012 a, b, Espinoza et al 2009, 2011,
2012, Tomasella et al 2010, 2012, Aragao et al 2007, and Coelho et al., 2012.
Figure 16 shows rainfall anomalies as derived from the Global Precipitation Climatology Centre Marengo et al 2008 data sets for three dry and three
wet years in Amazonia for the summer time peak rainfall season December- February. The main difference among dry years is the regional distribution
of negative rainfall anomalies across the region. In 1997-1998, negative rainfall anomalies covered almost all Amazonia, while in 2005 and 2010 the
anomalies were restricted to Southern and Northern Amazonia, respectively. In wet years, most of the regions with rainfall above normal
were detected in central Amazonia. This rainfall distribution pattern has consequences for the river discharge anomalies depending on the rainfall
patterns in the basins of the main Amazon rivers. However, changes in river levels are not proportional to the magnitude of the rainfall anomalies, and
in one or more sections of the Amazonian rivers, short or long-term changes in stream flow cannot be explained in terms of rainfall variability alone
Sternberg 1987, Marengo et al 2008, and Tomasella et al 2010.
Most of these extreme events were classified as such using river data statistics rather than on rainfall anomalies, considering that flood and
drought hazards represent the integrated impacts due to changes in rainfall across the basin. River data is perhaps the best indicator of impacts due to
excessive or deficient rainfall in the basin. At the Amazon main channel, or on the tributaries in the northern Rio Negro and southern basins
Solimões and Madeira Rivers, levels could vary in the same sense, or not, because rainfall anomalies may exhibit different spatial coverage.
Figure 17 shows a time series of the mean water levels of the Rio Negro at Manaus, for the peak season May-July. Since the levels of the Rio Negro at
Manaus show the impacts of rainfall over the Rio Negro basin in Northern- Central Amazonia and from the Solimões river basin in Southern Amazonia,
rainfall anomalies on those basins can vary from extreme to extreme, and show different impacts on the river levels. For instance, in 2005 and 2010
the drought was characterized by low rainfall in southwesternnorthern Amazonia and very low levels of the Madeiras and Solimões river Marengo
et al 2011 c, d, Tomasella et al 2012, but not as low levels at Manaus. In
contrast, in 1925, 1964, 1980 and 1983 the levels of the Rio Negro were below normal. The flooding in 1953, 1989, 1999, 2009 and 2012 also appears
on the figure well above normal. The river levels at Manaus allow for the detection of large periods with low river levels indicative of drought. Low
river levels were detected in the past in the 1910’s and 1920’s Marengo et al 2012a, b, but perhaps the best case study of a previous extreme drought in
Amazonia was that one of 1925-26 Meggers et al 1994, Williams et al 2005, when drought and fires killed many people.
The 2005 drought caused a simultaneous recession of the major tributaries of the Amazon river which led to a sharp fall in Amazon river runoff
Marengo et al 2008, Zeng et al 2008, Tomasella et al., 2010. Similar behaviour has been observed after the 2010 drought Marengo et al., 2011c,
d, 2012a, b. The impact of the 2005 and 1997-98 drought on floodplain communities was studied by Tomasella et al 2012 who found that since all
economic activities of these communities depend on the hydrological regime of the main stem they were heavily impacted by the droughts.
Their results revealed that the effects of the 2005 drought were exacerbated because
rainfall was lower and evaporation rates were higher at the peak of the dry season compared to the 1997 drought. This induced a more acute depletion
of water levels in floodplain lakes and was most likely associated with higher fish mortality rates Pinho et al, 2012. Based on the fact that the
stem growth of many floodplain species is related to the length of the non- flooded period, it is hypothesized that the 1997 drought had more positive
effects on floodplain forest growth than the 2005 drought. The fishing community of Silves in central Amazonia considered both droughts to have
been equally severe.
The 2009 flood event caused mudslides and drove nearly 200,000 people from their homes Marengo et al., 2011b and resulted in record discharge
being observed for the Amazon river. The record flooding in the Amazon in 2012 surpassed the previous record extreme in 2009, and river levels during
the drought 2005 and 2010 were among the lowest during the last 40 years Marengo et al 2012
c. In contrast, Brazilian newspapers and various government monitoring agencies reported that in 2012 the Amazon region
experienced one of the worst flooding episodes in history with most of the State of Amazonas under a state of emergency as rivers overflowed as an
emergency was declared in 52 of the 62 districts of this State. The rising
levels of the Solimões River and the Rio Negro, the two main branches of the Amazonia River, led to floods in both rural areas along the riverbanks and
in neighborhoods of the city of Manaus. Similar situations were observed in the rivers in the Peruvian, Colombian and Bolivian Amazonia, for both
drought and flood extremes.
Studies into these extreme events conclude that changes in the timing of positive and negative rainfall anomalies puts river discharges from the
northern and southern tributaries of the Amazon river in phase resulting in extreme positive and negative discharges whereas in normal years, the
timing is different attenuating the main-stem flood waves Tomasella, 2010; Marengo et al., 2011b, d, 2012a, b. Such unexpected and high magnitude
changes in water availability are likely to have a great impact on water security in the region, for transportation, agriculture and hydroelectric
generation. Hydropower potential is directly associated with discharge and therefore generally increases when forests are replaced with crops and
pastures because forests tend to release more vapor to the atmosphere through evapotranspiration, leaving less water for river discharge
Bruijnzeel 1991. Ecological impacts of extremes may affect the ecological functioning of trees; and large potential impacts on regional biogeochemical
and carbon cycles can be related to increase forest fires and biomass burning, as those observed during the droughts of 2005 and 2010 in
Amazonia. Lewis et al 2012 showed that while in most years the forests are a carbon sink, drought such as in 2005 and 2010 reverses this sink to
behave as a source.
There are limited quantitative results about the effects of changes in climate for human activities in the Amazon. The uncertainties in the
modelling of downscaling projections are still high, and further research on the effect of current extremes events is needed.
Figure 16 - Rainfall anomalies during December-February peak of the rainy season in Amazonia, in mmmonth, during dry years: 1997-98, 2004-2005 and 2009-10,
and wet years: 1998-99, 2008-2009 and 2011-12. Source of data is the Global Precipitation Climatology Centre See details in Marengo et al 2008.
Figure 17 - Time series of level anomalies mmmonth of the Rio Negro at Manaus since 1903, for the peak season May-July. Anomalies are in relation to the 1902-2012
mean. Dry and wet years are shown in red and blue colors, respectively.
5. Conclusions and Policy Options
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