RISKS Discover, every day, an analysis of our partner The Conversation. This Wednesday, a meteorologist reveals the climate future of this overseas territory franccedilaise-will-experience-more-droughts-and-marine-flooding-1ed2765.jpg” alt=”Climate change: French Polynesia will experience more droughts and marine flooding”/>
French Polynesia has 120 islands united in 5 archipelagos according to which the climate varies. It is therefore complicated to measure the consequences of global warming on the whole region — Shutterstock/The Conversation
- Analysis of long series of temperature data suggests that the Polynesian climate has warmed up in recent decades, according to our partner The Conversation.
- In French Polynesia, the first rainfall measurements date back to elsewhere in 1853 and the first meteorological station located on the heights of Mont Faiere, in Sainte Amélie, was inaugurated in 1935.
- This analysis was conducted by Victoire Laurent, meteorologist.
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To observe climate change, the World Meteorological Organization recommends assessing climate change over a period of at least 50 years in order to overcome biases linked to ;s to other temporal climate variability, such as climate drift, which is calculated over 30 years, the ENSO phenomenon (El Niño Southern Oscillation), which modifies the climate over a period of 30 years; scale from 2 to 7 years, or the Pacific Interdecadal Oscillation (IPO), which varies over 40 years and has a lasting influence on rainfall in French Polynesia.
The community climatologists has therefore embarked on a vast data backup program in order to compile long series of rainfall and temperature data. In French Polynesia, the first rainfall measurements date back to 1853 and the first meteorological station established on the heights of Mont Faiere, at Sainte Amélie, was inaugurated in 1935.
After having inventoried and digitized old data, it is necessary to standardize these long series of data to eliminate errors or breaks due to changes in sensors or measurement conditions. À From these long homogenized and reliable series, we seek to detect the signal of climate change and quantify it.
The archives of the climate service of Météo-France in French Polynesia – Victoire Laurent/The Conversation
Signs of climate change
French Polynesia, which has 120 islands divided into 5 archipelagos, has a humid tropical climate but with notable differences due to the large latitudinal extent of this territory (more than 20° from North to south). We identify from an archipelago to the other a very specific climate: hot and dry in the Marquesas Islands, but cool and humid in the Austral Islands, or even wetter in the Société than on the Tuamotus.
By analyzing the long series of temperature data, we can say that the Polynesian climate has warmed up. over the past few decades. Depending on the archipelagos, we calculate an average rise between +0.6 °C and +1.55 °C consistent with global warming over this period.
This increase in temperature is greater at night than during the day. Thus at the weather station of Faaa (Tahiti), the average rise in minimum temperatures, usually observed at the end of the night, is practically twice as high as the rise average of the maximum temperatures, generally observed during the day, with respectively +2 °C and +1.3 °C in 58 years. This climate change also has an impact on heat waves, the total number of cases of which has increased significantly. between 1964 and 2021.
Contrary to what one might think, there is little impact of climate change on the annual cumulative rainfall, apart from isolated stations such as Omoa, the Marquesas rainfall station, and Rapa, the meteorological station of the Australs. This lack of rainfall signal of climate change is also confirmed on other islands in the South Pacific. On the Polynesian territory, the linear rainfall trends calculated over a minimum period of 50 years vary on average between +5 mm/year and -6 mm/year, but these variations are significant. ; explain by influences other than climate change, such as the IPO and ENSO mentioned above.
The activity hurricane is considered weak on average over the Polynesian basin. In fifty years, we have counted 23 tropical cyclones, 17 strong tropical depressions and 10 moderate depressions. The last cyclone that affected French Polynesia is Oli in February 2010, with a minimum pressure of 937 hPa (the hectopascal is the unit of measurement for atmospheric pressure) and a maximum wind of 250 km/h, far from the records held by the tropical cyclone Orama of February 1983, with a minimum pressure of 898 hPa and a maximum wind of 280 km/h.
The analysis since 1970 of the activity of cyclone shows that it has been decreasing over the last two decades and that this decrease is more marked for tropical cyclones.
DTM (Moderate Tropical Depression), DTF (Severe Tropical Depression), CT (Tropical Cyclone) – Victoire Laurent/The Conversation
The recently produced and analyzed climate projections maintain this decrease in activity for the end of the century. over the South Pacific and do not show a clear signal of a change in the frequency of category 5 cyclones (in the Pacific, category 5 corresponds to intense tropical cyclones with medium winds over 10 minutes greater than or equal to 166 km/h). However, simulations show an increase in rainfall within a radius of 200 km around the center of the cyclone.
At ocean level, unlike atmosphere, climatologists do not have long data series to quantify the effect of climate change even if since the 1990s, ocean information is more important thanks to this to data collected by satellites. To quantify the impact on sea surface temperatures and sea level rise in French Polynesia, reference can be made to climate trends.
Like air temperatures, surface waters in the Pacific Ocean have warmed by +0.9°C since 1982, contributing to sea level rise by expansion.
Globally, sea level is rising more rapidly over the past few decades, but at a slower pace. different speeds depending on the archipelagos. According to the data from the LEGOS/CLS satellite images, it is noted that since 1992, the sea level has increased. to an average speed of +2.9 mm/year at Tahiti and from +1 mm/year to Mangareva, trends that would lead us in 2050 to much larger increases than those proposed in the 5th IPCC report.
The climatological atlas of French Polynesia, released at the end of 2019, devotes an entire chapter to climate change and, recently, a report on the state of knowledge of the climate of the Pacific islands and its impact. ;evolution has been uploaded.
Projections and future plans
In the current state of our knowledge, it can reasonably be argued that climate change in French Polynesia has a negative effect. the 2050 horizon will lead to a significant rise in temperatures and a decrease in the amounts of rainfall on some islands, increasing the vulnerability of the islands. to availability of water. The risk of wildfires on high islands could increase with more frequent and longer periods of drought.
At the ocean level, the main vulnerability is generated by climate change is the increased risk of marine submersion. À like what happened on the Tikehau atoll during the passage of the strong swell of 1996, the structures by the sea would be more vulnerable to the risk of submersion.
Tikehau airport (Tuamotu), submerged by the strong swell of 1996 – Victoire Laurent/The Conversation
Depending on the morphology of the islands and the temperature of the ocean, the coasts exposed to extreme swells would be threatened by the rise in sea level. For the atolls, these marine submersions would result in salinization and therefore the contamination of freshwater lenses, weakening their safety; food and health.
À On the scale of the islands of French Polynesia, we do not have all the answers on the evolution of the climate for the end of this century. Since the IPCC projections are made at 100 km resolution, islands are not shown. It’s on this issue that the CLIPSSA project (CLImat of the Pacific Local Knowledge and Adaptation Strategy) began. in 2021, for a period of three years.
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Using downscaling methods, the actors of this project will produce climate simulations at scale. scale of 2.5 km, in order to respond to the problems at hand. long and very long term of local actors, and thus allow the implementation of adaptation strategies taking into account the cultural specificities of these islands.
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< em>This analysis was written by Victoire Laurent, meteorologist, head of the studies and climatology division of Météo-France (Interregional directorate for French Polynesia), Université of French Polynesia.
Original article was posted in published on The Conversation site.
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