SDGAcademyX: AMZN001 The Living Amazon: Science, Cultures and Sustainability in Practice

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2. Hi, my name is José Antonio Marengo. I'm a
3. climatologist, I work with research and
4. development at the Brazilian National
5. Center for Monitoring and Early Warning of
6. Natural Disasters in São Paulo. And in this
7. chapter, we're going to talk about long-term
8. change in hydrology. We describe the
9. observed change we have noticed, and also
10. the projections we have in precipitation
11. and river discharge in the Amazon region,
12. according to different warming
13. scenarios. And also the historical trends
14. in the Amazon precipitation vary
15. considerably depending on the studies
16. (similar in the case of temperature), and
17. also depend on the data set, the period
18. of the data, and also the season,
19. because one thing is wet season, the other
20. thing will be different than dry season.
21. Also, extremes——the extremes of interannual
22. variability and
23. rivers in the Amazon region can be attributed
24. in part to anomalies in sea surface
25. temperature, variations in sea surface
26. temperatures in the tropical oceans,
27. particularly the Pacific and the Atlantic.
28. And in the case of the Pacific, we have
29. El Niño Southern Oscillation (ENSO). And also, we have
30. the meridional sea surface temperature
31. gradient in the tropical Atlantic,
32. basically what we call "TNA," which is the
33. northern and the southern contrast, which
34. is extremely important in terms of the
35. position of the system that produces rainfall.
36. While we have not noticed a long-term
37. unidirectional trend in annual rainfall——
38. meaning that we don't notice a general
39. increase or decrease——what we notice is
40. some regional changes. The situation
41. may be different according to the region
42. and also the seasonal levels, so
43. long-term, decadal
44. variations linked to natural
45. climate variability are always present,
46. and they have influence in rainfall
47. trends because most of the rainfall
48. records are over the Amazon. And the only
49. available data in terms of precipitation
50. is for the recent decades; that's the
51. reason why we use most of the river data as
52. an indicator. Studies analyzing rainfall
53. in the Amazon over the past four decades, they
54. show contrasting north-south trends:
55. The northern part with increasing
56. rainfall, and the southern part with
57. decreasing
58. rainfall. Of course, it depends on the
59. data we use for these studies, but what is
60. extremely important is that the recent
61. analysis reinforced the trend towards
62. negative
63. rainfall in the southern part of the
64. Amazon. This happens particularly
65. during the wet season,
66. you see. And due to the higher rainfall
67. in the northern
68. Amazon, the precipitation has
69. been increasing. The northern part
70. increases by about 2.5, 2.8 mm per year
71. during
72. 1981 to
73. 2017. These changes are related to
74. changes that happen in the atmosphere,
75. what we call the "decadal changes." It
76. happens between 20 to 30 years. The
77. Amazon precipitation has been
78. affected by this. Some of these changes
79. are what called the "Pacific Decadal
80. Oscillation," then the "Inter-Decadal
81. Pacific Oscillation" in the Pacific
82. sector, and then the "Atlantic Multi-Decadal
83. Oscillation" in the tropical Atlantic.
84. We have to remember that climate in the
85. Amazon is governed by both oceans. And
86. these decadal rainfall fluctuations over
87. the western Amazon vary closely with
88. this gradient between the northern part
89. of the southern part of the sub-tropical
90. Atlantic. For standardized anomalies
91. of monthly mean rainfall from the Amazon
92. region, we have this figure produced
93. with the CHIRPS data, which is a
94. combination of satellite and
95. observations. The bars over the positive
96. side actually are above zero, and then
97. which shows more rainfall, and then the
98. bars on the negative side, which shows
99. less rainfall. In some of these cases,
100. this less rainfall has been leading to
101. droughts. And when we see those fat arrows,
102. they represent the presence of
103. drought, you see? And this has been
104. happening in different periods like
105. interannual, what we call El Niño, for instance,
106. and then in the longer time scale,
107. because the analysis was 1980-2020
108. because of the data, OK? But if we had
109. more data, we could be able to
110. identify most most of these peaks. But,
111. the decrease of rainfall in the southern
112. part of the Peruvian Amazon——Peruvian,
113. Brazilian, and Bolivian Amazon, for
114. instance——is most particular during the
115. dry season. And it has been associated
116. with the onset of the South America
117. monsoon. The
118. Amazon climate is part of what we
119. call the South American Monsoon System (SAMS), and
120. a late onset means that we are going to
121. have a relatively dry monsoon. And in
122. those cases, we have what in atmospheric
123. terms is called "subsidence," which means, basically,
124. air descending and this reduces
125. the possibility of rainfall. All of these
126. changes have been documented.
127. Several studies have been showing that
128. this increase of the dry season has been
129. noticed in the southern part of the Amazon
130. since the 70s, since the beginning of the
131. 70s, you see? And this has, perhaps, been
132. more obvious during dry years——2005, 2010,
133. and
134. 2016, as well as in previous droughts
135. effects. If the rainy season starts
136. late, the dry season is longer; we have
137. impacts on the river levels and we have
138. also high risk of fire, OK? In the case
139. of the Rio Negro, since the Rio Negro
140. levels in Manaus started in 1902, perhaps,
141. this is the best indicator of climate
142. variability we have found in the region,
143. OK? And this show, for instance, changes
144. in the hydrometeorological extremes. Some
145. of the severe droughts in the Amazon
146. region——like in 2005, for instance——are not
147. related to El Niño, you see, but typically, we
148. say, typically, during El Niño years, we have
149. droughts; like, it happened in 2010, 1983,
150. 1988, and more recently in
151. 2012 and 2016. You see? And some of
152. these droughts have also happened
153. because of the southwestern part of the
154. Amazon has drought and also because
155. the tropical North Atlantic is
156. warming. In fact, during the last 20 years,
157. three megadroughts——like 2005, 2010, and
158. 2015-16——were classified at the time as a
159. one-in-100-year event, you see?
160. And all of them were related to El Niño.
161. When we have megaflows like, 2009,
162. 2012, and
163. 2021, are related to La Niña, which is the
164. opposite of El Niño, you see? And then some of the
165. cities really are not prepared. For
166. instance, in
167. 2023, we have plenty of rainfall in there,
168. and we have major problems with flows in
169. bigger cities, including Manaus and Belém. So,
170. looking as we saw in temperature with
171. the projections of the CMIP5 models, you
172. see,
173. depending on the year, we have a rainfall
174. reduction in the southern part of the
175. Amazon. This rainfall reduction is most
176. particular during September and October,
177. meaning, again, the late onset of the
178. rainy season. So, we have a late onset of
179. the rainy season, and rainy season may
180. have been weaker. And we have this shown
181. for different periods, from 2022 to 2090
182. based on the 1961-2005 baseline. And a
183. warmer and dryer Amazon in the future——this
184. is a major question we have, what could
185. happen? And the Amazon region may become
186. warmer or dryer, as shown by the models,
187. particularly in the central and eastern
188. part of the Amazon, where we have the
189. Deforestation Arc. They're approaching a
190. catastrophic situation with droughts,
191. floods, and the risk of fire, and impacts
192. on population and biodiversity could be
193. really happening more continuously.
194. Regional climate change scenarios for
195. rainfall, for instance, derived from the
196. downscaling of the English model HadGEM2
197. with the Brazilian Eta model. This
198. shows for 2041-2070, relative to
199. 1961, reductions of rainfall in
200. several parts of the Amazon. And this
201. reduction of rainfall also reflects in
202. the reduction of the discharge or stream
203. flow of the rivers. In the figure, we have,
204. for instance, three rivers: the different
205. patches [colors] on the circles, they show the
206. Madeira,
207. Tapajós, and Xingu Rivers. The size of the
208. circle shows the magnitude of this change.
209. Massing all of them, you see in the x
210. coordinate negative changes in
211. precipitation, and in the y coordinate,
212. positive changes in temperatures, meaning
213. that warmer climates show a reduction
214. of rainfall and positive reductions on
215. stream flow. Again, the similar situation
216. for the Eta model and the HadGEM2,
217. this model produced by Brazil,
218. actually, they have been showing the
219. kind of change. So, what we can say in
220. summary is that even though we don't
221. have a tendency for a unique trend of
222. rainfall in the whole Amazon——meaning
223. that the Amazon is getting drier or wetter——
224. we have regions that show that
225. more rainfall, particular to the northern
226. part, less rainfall, particular to the
227. southern part. And something which is
228. extremely important is this combination
229. of temperatures: higher temperatures and
230. less rainfall, it will have a strong
231. impact on the quality of the rivers,
232. meaning, basically, the low river levels,
233. this affects population, particularly
234. what we call the "ribeirinhos," which are the
235. people living on the banks of the river and
236. depending on the river for everything,
237. basically.
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