Information Sheet on Future Climate and Impacts in the Coastal Case Studies: Gulf of Gabès

Information Sheet on Future Climate and impacts in the coastal case studies: Gulf of Gabès, Tunisia

Summary

►Mean air temperature over the Gulf of Gabès is projected to increaseduring the period 1950-2050. Daily minimum and maximum temperatures show a similar increase. Precipitation is decreasingslightly. The simulated trends are very weak before the year 2000 and the rate of increase acceleratesthereafter. There are no significant trends for wind speed and relative humidity.

►The Gulf of Gabès waters are warming, also at higher rates after the year 2000. Salinity is increasing at a very low rate. There is evidence for salinity decrease at long time-scales. Sea level is increasing mainly due to the steric effect.

►As a consequence of water warming alien marine species are projected to increase.

►Similar to observations, the summer season is projected to lengthen leading to an extended “touristseason”.

►Similar to observations,the number of days “favourable” for tourism activities deduced from climate projections, decreases whereas the number of those just “acceptable” increases. Such changes are accentuated in summer; in contrast the number of “favourable” days increasesin winter.

►Socio economic impacts of changes in climateconditions on tourism activity are estimated based on model simulations. They show that impacts increase after the year 2000.

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1. Introduction

This information sheet is the third one in the series on the present and future impacts of climate change over the Gulf of Gabès. The first information sheet focused on observed climate-marine conditions (Harzallah, 2010). It showed that mean air temperature is increasing over the Gulf of Gabès. Daily maximum air temperature is also increasing particularly in summer and autumn. Rainfall showed no clear trends. The summer season has lengthened over the observation period with a tendency for an earlier start date and a later end date. The second information sheet (Harzallah et al., 2010) showed important shoreline modifications along the Gulf of Gabès coasts, although these were mainly related to human activity. Anincrease inthe annual maximum swell height deduced from wind statistics together with the observed sea-level rise may reinforce the vulnerability of the coasts. The number of alien marine species discovered in the Gulf is found to increase, which may negativelyimpact biodiversity inthe Gulf and possibly the fishing activity in the region. The tourism sector may be impacted by beach erosion and by the increase of ‘favourable’ daysin winter and theirdecrease in summer.

This information sheet shows climate projectionsfor the present and future periods in theGulf of Gabès(1950-2050), based on the most recent set of model simulationsavailable, including for the first time a coupling of the Mediterranean Sea with the atmosphere. The set of atmospheric and oceanic variables simulated by the models are presented and compared to the observed ones whereavailable. Biogeophysical impacts are investigated along with some economic and social future trends.

Indicators are presented for:

►Climate conditions

►Marine conditions

►Alien marine species

►Seasonal shift Index

►Daily Climate Tourism Potential Index.

►Socio-economic tourism impacts.

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Climate models

The CIRCE project climate model runs (see Information sheet on future climate: Climate projections for the CIRCE case studies, are used to investigate present and future climate changes in the Gulf of Gabès for the period,1950-2050. After the year 2000, the A1B scenario for the GHG aerosols concentration is used(Nakićenović et al, 2000).The CIRCE climate models are of differing spatial resolution and the areas covered for the Gulf of Gabèsare shown in Figure 1. The following coupled atmosphere-ocean CIRCE models are used:ENEA; ENEA-ERA40-2(using observation reanalyses for the period 1958-2000); INGV; LMDglo (also referred to as IPSL1 elsewhere); MF (also referred to as CNRM elsewhere);and MPI.

Figure 1: Gulf of Gabèsareas covered by the atmospheric and oceanic components of the CIRCE climate models. Left (atmosphere); right (ocean)

In addition, two ocean model runs (INSTMED06 and INSTMCOTR) are used for the three 10-year periods (1960-1969, 1991-2000 and 2050-2059).

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2. Climate and marine projections

Climate conditions

What is it?

Figure 2 shows key weather variables in the Gulf of Gabès simulated by the ensemble of CIRCE models for the period, 1950-2050. Mean values and trends during the ‘present climate’, 1961-1990 and the‘mid-century’, 2021-2050 are shown in Table 1. Also shown are the‘long-term’, mean changes and mean change rates between these two periods. The Mann-Kendall test (Mann, 1945; Kendall, 1975) and the student-t test are used to asses the statistical significance of trends and of the mean changes (andmean change rates). These tests are also used to asses the statistical significance of the subsequent series shown in the present information sheet.

Figure 2: Simulated atmospheric variables in the Gulf of Gabès (averaged over the areas shown in Figure 1) from a set of numerical models for the period, 1950-2050. The black line is the average series based on the different simulations (except ENA-ERA40-2). Details on models and simulations are shown above. The variables are mean, minimum and maximum air temperature (2m), precipitation, wind speed and relative humidity

Table 1: Mean values and trends of key variables simulated by the five coupled models in the Gulf of Gabès area (Figure 1) based on ensemble averages, for the present 1961-1990 and mid-century 2021-2050 periods. Mean long-term changesbetween the periods 1961-1990 and 2021-2050 are also shown. Inter-model ranges are shown as the standard deviations between estimates from the five models.Bold values are significant.

1: Inter-model Mann-Kendall trend test: the null hypothesis of an absence of trend is rejected at the 0.05 significance level; 0: a failure to reject the null hypothesis.

1: Inter-model Student’s t-test: the null hypothesis means are equal is rejected at the 0.05 significance level;0: a failure to reject the null hypothesis.

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2. Climate and marine projections

What does this show?

Mean air temperature is projected to increase at an accelerating rate. The different models show similar behaviour. The observed annual temperature for the period 1961-1990 from data archived at the Goddard Institute for Space Studies (GISS, range from 19 to 20°C at Gabès and 18-19°C at Sfax. The coupled models slightly underestimate the temperature in the Gulf of Gabès (a general characteristic of the CIRCE simulations showing a cool bias of around -2°C over lands). However, model data cover the areas shown in Figure 1, whereas observations are for stations. The simulated trends are also slightly less than the observed ones and show that the mean, minimum and maximum temperatures increase by around 1.4 °C between 1961-1990 and 2021-2050 and 2°C from 1950 to 2050. Temperature trends are very weak during the ‘present’ period and are strongerand significant during the ‘mid-century’ period. The simulated precipitation series show a generally declining trend after the year 2000; however, trends are not significant. In addition, wind speed and relative humidity show no clear trends.

Why is it relevant?

The simulated trends inmean temperature for ’present’ climate conditions are lower than observed ones (+0.013°C/decade simulated in 1961-1990 versus +0.26°C/decade for the 1948-2008 and +0.46°C/decade for the period 1973-2008, from the GISS archive at Gabès station) but it is important to remember that modelprojections are for climate changes due to observed and projected greenhouse gasconcentration under the A1B scenario. Part of the stronger trends observed may result from decadal variability which is not necessarily reproduced by the models.

More importantly, the CIRCE simulations show for the first time projections for the future situation based on multi-model outputs with the atmospheric component coupled to the Mediterranean Sea at a high resolution. There is a clear acceleration of the changes during the 21st century most prominent for the air temperature. The CIRCE models project more severe conditions with a warmer climateand slightly reduced precipitation.

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2. Climate and marine projections

Marine conditions

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2. Climate and marine projections

What is it?

Annual mean sea surface temperature (SST) averaged over the Gulf of Gabès area from the five CIRCE coupled models (and the ENEA-ERA40-2 model) are shown in Figure 3 for the period 1950-2050. In addition Figure 3 shows the SST from the two forced ocean models (the Mediterranean Sea model: INSTMED06, and the higher resolution regional model: INSTMCOTR) for the three simulated periods 1960-1969, 1991-2000 and 2050-2059. Figure 4 shows sea surface salinity (SSS) simulated by the coupled models and the average series for the period,1950-2050.

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2. Climate and marine projections

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2. Climate and marine projections

Table 2a shows the mean values and trends for SST and SSS for the periods 1961-1990 (present) and 2021-2050 (mid-century). It also shows the rate of change between these two periods (‘long-term’ changes). The inter-model ranges are shown as the standard deviations between estimates from the five models. Table 2b shows the rates of change for SST, SSS and SSH (sea surface height) between the three simulated periods by the INSTMED06 and INSTMCOTR forced models. For the INSTMCOTR model SSH represents the contribution from water circulation with reference to the central Mediterranean. For the INSTMED06 model SSH represents the effect of volume expansion and contraction due to the variation intemperature and salinity (the steric effect).

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2. Climate and marine projections

Table 2a: Mean values and trends in SST and SSS from five CIRCE coupled models in the Gulf of Gabès, based on series averages for two periods, 1961-1990 (present) and 2021-2050 (mid-century). Mean changes and mean rates of change between the present and mid-century periods (long-term) are shown. Inter-model ranges are shown as standard deviations between estimates from the five models.Bold values are significant.

1: Inter-model Mann-Kendall trend test: the null hypothesis of an absence of trend is rejected at the 0.05 significance level; 0: a failure to reject the null hypothesis.1: Inter-model Student’s t-test: the null hypothesis means are equal is rejected at the 0.05 significance level;0: a failure to reject the null hypothesis.

Table 2b: Mean rates of change for SST, SSS and SSH between the indicated periods from the forced models INSTMED06 (left values) and INSTMCOTR (right values). Bold values are significant.

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2. Climate and marine projections

*SSH for the INSTMED06 model is relative to the steric effect in the Mediterranean basin; **SSH for the INSTMCOTR model is relative to the part related to the ocean circulation with a reference in thecentral Mediterranean.1Student’s t-test: The null hypothesis means are equal is rejected at the 0.05 significance level;0a failure to reject the null hypothesis.

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2. Climate and marine projections

What does this show?

Projectedwater temperatures are lower than expected for the Gulf of Gabès where observations show an average value of 20.6°C. Nevertheless all models show an increase in SST with acceleration during the 21st century. SSS shows weak trends, although there is a tendency for a slight lowering of salinity at the long-term time-scale.ForSSHthe change is mainly due to the steric effect, +0.5 cm/decade between the periods 1960/1969 and 1991/2000, and +2.5 cm/decade between the periods 1991/2000 and 2050/2059. The observed sea-level trends are +2.6 cm/decade for the period 1999-2007 from data provided by the ‘Centre d’Hydrographie et d’Océanographie de la Marine Nationale, Ministère de la Défense Nationale’, Tunisia at Sfax; +2.1 cm/decade is observed by altimetry data in the Mediterranean basin for the period 1992-2005, Criado-Aldeanueva et al., 2008). Estimates of the steric component are also obtained from the coupled models but at the Mediterranean basin scale (not shown). Relative to the period 1961-1990, the changes projected are +10cm forthe period, 2021-2050 and +14cm for 2050. The steric component accounts for approximately 70% of total sea-level change (IPCC 2007); the projected sea-level rise is +14cm forthe period 2021-2050 and +20cm for 2050. A similar estimate for 2050-2059 relative to 1960-1969 (INSTMED06 model, Table 2b) leads to +23cm.

Why is it relevant?

Similar to the atmospheric variables, the oceanic ones show stronger trends after the year 2000. For the period 2021-2050 models project +0.54°C/decade and +0.42°C/decade for air and sea temperatures trends respectively. These close values again reflect how fast the shallow gulf responds to air warming. The projected water warming reinforces the expected changes in the gulf ecosystem including changes in biodiversity, fish resources with ‘knock-on’ impacts on the fishing activity in the region. The very weak change in projected salinity suggeststhat salinity will probably not constitute an additional constraint on the gulf ecosystem. However a sea-level rise of nearly +20 cm is projected in 2050 which couldaccentuatethe vulnerability of the gulf to erosion and inundation.

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2. Climate and marine projections

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3. Future impacts and vulnerabilities

Alien marine species

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3. Future impacts and vulnerabilities

What is it?

Harzallah et al., (2010) showed that the number of alien species isincreasingin the Gulf of Gabès at a rate of 15% decade when corrected for bias due to the increasing number of studies (on alien species). This increase is attributed to changes in marine water characteristics (mainly increase in water temperature). The cumulated number of new observations corrected for the increasing number of studies is shown in Figure 5 together with anexponential fit. The reference date for the corrected series and exponential fit is the year 2007 with 40 new observations. With the hypothesis that alien species are related to the SST increase, a linear transformation is performed on the average SST projected bythe different models shown in Figure 3 (thick line), so that the transformed SST equals the fitted exponential at two dates, 1966 and 2007 (first and last observation dates). The fitted SST gives the linear transformation neededto obtain an estimate of the new species from SST: Fitted SST=42.8*SST-741.6. The cumulated number of new species increases with SST at a rate of 42.8%/°C.

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3. Future impacts and vulnerabilities

Figure 5: Estimated number of alien species in the Gulf of Gabès(red) obtained as a fitted exponential functionto SST (Fitted SST=42.8*SST-741.6). An exponential fit (blue line) to observations with a trend of 15%/decade is shown.The observations (blue dots) are cumulative number of new species, (see Harzallah et al. 2010)and are adjusted to correct for the increasing number of studies with a reference the year 2007.

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3. Future impacts and vulnerabilities

What does this show?

Estimates of future behaviour in alien species can be estimated using numerical simulations based on the hypothesis that alien species observed in the Gulf of Gabès are related to sea warming. The projected increasing trend inalienspecies corresponds to 42.8%/°C.

Why is it relevant?

In 2007 the total number of new species is estimated as 40. During the period 2007-2050 an increase inthe sea-surface temperature of 1.6°C is projected (see Figure 3). Using the above linear transformation the total number of new species is estimated as nearly 106 in 2050. Such a large number of new species wouldconstitute an additionalstress on the gulf ecosystem in particular for its biodiversity. It is therefore important to considerhow the ecosystem wouldrespond to such a large number of projected new species. However, it is unlikely that temperature warming is the only driver of this increase in invasive species. Their occurrence is more likely resulting from a combination of different mechanisms, ecological, other environmental process and human activity.

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3. Future impacts and vulnerabilities

Seasonal Shift Index

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3. Future impacts and vulnerabilities

What is it?

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3. Future impacts and vulnerabilities

Using an index for the summer season extension, Harzallah et al. (2010) showed that there has been a lengthening of this season with an earlier start and a later end during the period 1960–2008. The devised Seasonal Shift Index, measures the start and the end of the summer season, occurring when the observed daily mean air temperature reaches 23°C for the first time. The index is similarly constructed using model data for the period 1950-2050, and is shown in Figure 6. The index is constructed for each model. An average index is then obtained. Table 3 shows the dates and trends calculated for the start, the end and the duration of the summer season for the ‘present’ 1961-1990 and ‘mid-century’ 2021-2050 periods. It also shows the corresponding ‘long-term’ mean changes and mean rates of change between these two periods. Prior to calculations, the average temperature series foreach model for the period 1973-2003 is adjusted to the mean observed temperature at Djerba during the same period. This technique helps to correct for bias between model data and observations in the calculation of the start and end dates.

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3. Future impacts and vulnerabilities