Location of the study area

Tabasco is located in the southeastern region of Mexico between 18° 38' and 17° 15' north latitude and 90° 38' and 94° 07' west longitude. Neighboring to the north with the Gulf of Mexico, south of the state of Chiapas, west with Veracruz, northeast with Campeche and southeast with Guatemala. It has an area of 24 751 km² integrated by 17 municipalities from five regions: the Chontalpa (which includes the municipalities of Huimanguillo, Cardenas, Comalcalco, Paraíso, Cunduacán), Sierra (which includes the municipalities of Teapa, Tacotalpa and Jalapa), los Pantanos (Macuspana, Jonuta and Centla), los rios (formed by the municipalities of Balancán, Tenosique and Emiliano Zapata) and el Centro (formed by the towns of Nacajuca, Jalpa de Mendez and Villahermosa) (^{INEGI, 2015}).

Selected weather stations

Daily data of maximum and minimum temperature and precipitation for a period of 50 years (1961-2010) from five representative localities, one for each region were used. The daily data were extracted from the database contained in Eric III v.3.2 program which contains the information of the national historic data bank from the National Weather Service (^{IMTA, 2013}). In Table 1 are listed the five selected weather stations from the state of Tabasco, which were used for this study. Also, describes information from the station like code, latitude, longitude and meters above sea level.

Table 1 Representative weather stations of each of the five regions from the state of Tabasco. 

Procedure in data analysis

For the analysis of the possible change of trend in temperature and precipitation in the five selected localities, the following steps are performed:

Changing trends in the annual average maximum temperature

Figure 1 shows that during the period 1961-2010 in the state of Tabasco, the maximum annual temperature increased on average 0.0222 °C per year, which is equivalent to an average increase of 1.1 °C in that 50 year period. To continue this trend in the future, for 2050 and 2100, increases expected in maximum temperature of 2 °C and 3.1 °C respectively. sAlso in Figure 2 can be observed that in the five regions positive trends occurred during the period 1961 to 2010 ranging from 0.2 °C for Chontalpa, up to 3.3 °C for los Rios. The regions that had less increase in their maximum temperatures were Chontalpa and Centro with 0.2 °C and 0.3 °C respectively; while the largest increases occurred in the region los Pantanos with 1.3 °C and Rios with 3.3 °C.

It is worth noting that increases in the latter two regions are very high and significant, as it is equivalent to the increases that assembled models have proposed for average temperature for the horizons 2050 and 2100 (^{Conde and Gay, 2008}). That is, that locally the warming process in these two regions has accelerated regarding the average maximum temperature. Also note that the different global and regional scenarios that have been developed make their increase estimates in average temperature and nothing is said about trends of average annual maximum and minimum temperatures.

Trend change in average annual minimum temperature

Regarding the average annual minimum temperature from Tabasco are shown in Figure 2, during the period 1961 to 2010 there was a positive trend with a rate of 0.0114 °C per year, equivalent to an increase of 0.57 °C during this period. If the same trend continues, there would be increases of 1.05 to 1.6 °C for the horizons 2050 and 2100 respectively. Comparing Figure 1 and 2 can be seen that on average the maximum temperature at state level has increased almost twice than minimum temperature.

In Table 2, it becomes clear that among regions, there were positive and negative differences regarding trend change in the average annual minimum temperature. So while the Sierra region had a negative trend with a decrease of 0.2 °C, the rest of the regions showed positive trends with increases ranging from 0.5 °C for Pantanos and 1.1 °C for Centro. This means that nights have been warming during the study period for those four regions. Probably the albedo of the City of Villahermosa and flooding condition in Pantanos region explain the differences in the positive trends. Similarly, it is possible that the location of the Sierra region explain the cooling that has occurred during the study period. The Rios region had a significant increase of almost one degree (0.8 °C), ranking second in increases in the minimum annual temperature in the state of Tabasco.

Trend change in annual average temperature

Figure 3 shows that the trend in average temperature in the state of Tabasco was positive during the period 19612010, with an average increase of 0.86 °C. If the same trend continues, it is expected for 2050 and 2100 that the average temperature will increase 1.54 and 2.39 °C respectively. That is, that by the end of the century it would be expected an average increase in average temperature of 2.39 °C. The observed increase in Tabasco is slightly above the world average observed in the average global temperature as reported by the ^{IPCC (2007)}.

Also, expected increases are consistent with the resulting maps for Mexico from the simulation of 23 Atmosphere-Ocean General Circulation Model reported by ^{Conde and Gay (2008)}. These results indicate that by year 2050 and for the state of Tabasco, an increase between 0.8 to 2.4 °C is expected for the best and worst case scenario (B1 and A2); and an increase between 1.0 and 3.8 °C by 2100 for B1 and A2 scenarios respectively. Table 3 shows that the five regions show a positive trend change in average temperature, with increases ranging between 0.3 °C for Sierra region and 2.1 °C for Rios.

In the last region, the increase is strongly influenced by the increases observed in the maximum temperature; as can be seen in Table 4. If the trend continues in the increase of average temperature observed in Tabasco allows confirming that some plant and animal species can adapt to new conditions, even some of restricted distribution could extend their habitat as reported by ^{Arriaga and Gómez (2005)}.

Trend change in annual average atmospheric temperature range

The atmospheric temperature range (OSC) is the difference between the maximum minus the minimum temperature.

Analyzing the oscillations along 50 years allows discriminating whether the change in annual average temperature is primarily due to increases in maximum temperature or to minimum temperature. That is, if the warming is due to the day is being heated more than the night or that the night it being heated more than the day. Thus, a decrease in atmospheric temperature range may mean that the minimum temperature has been increasing more than the maximum temperature while an increase in the atmospheric temperature range can mean that the maximum temperature has been increasing more than the minimum temperature. Figure 4, shows that for the state of Tabasco, the annual average atmospheric temperature range has a periodic behavior with a negative trend period (1961-1984) and another with positive trend (1984-2010); and with an overall positive trend, with an average increase of 0.0114 °C per year, equivalent to an increase of 0.6 °C for a 50 year period.

This can be explained by observing the average values of the maximum and minimum temperature from the state of Tabasco in Figures 1 and 2. In these figures, it is shown that the maximum temperature has been increasing on average, almost twice than the minimum temperature. I.e., the increase of atmospheric temperature range over time is due to higher increase in the maximum temperature. These results at state and regional level contradict that mentioned by the IPCC globally; arguing that global warming is explain by a higher increase in night temperatures (^{IPCC, 2007} and ²⁰¹²).

In Table 5, it can be seen that at the regional level, there were opposite trends. While in the Chontalpa and Centro region the trend was negative, in the rest of the regions was positive. Thus, in Chontalpa and Centro region, the atmospheric temperature range decreased -0.6 °C and -0.9 °C respectively, because the average minimum temperature rose almost 3 and 4 times more than the maximum temperature, as seen in Figure 4. That is, in Chontalpa and Centro, the nights have been warming more than the day. This situation is consistent with that indicated by the Intergovernmental Panel on Climate Change (^{IPCC, 2007}, ²⁰¹³). But for Sierra, Pantanos and Rios region the atmospheric temperature range show a positive trend of 0.8, 1.3 and 3.3 °C respectively. Tables 2 and 3, show that in these regions the maximum temperature during the study period has increased almost 5, 3 and 4 times more than the minimum temperature. That is, that the day has been warming more than the night.

Trend change in average annual total precipitation

The scenarios indicate trend change for precipitation, anomalies reported in percentage of total annual precipitation in deviations regarding the mean (IPPC, 2007). Importantly, the results of the trend change in average annual total precipitation in the state of Tabasco had minimal anomaly of 0.5% during the study period (1961-2010). This means that the state average amount of precipitation did not vary during that time. This is shown in Figure 5, where the annual increase rate was 0.21 mm year^-1, with an accumulation in the 50-years study of 10.7 mm.

Figure 5 Trend change in annual average total precipitation in the state of Tabasco, during the period (1961-2010). 

If the same trend continues, it would be expected by 2050 and 2100 an average increase in annual total precipitation of 19.2 and 29.9 mm respectively; negligible values compared to annual total. However, at regional level there were differences in trend change, as the Sierra region showed a negative trend, while the rest of the regions it was positive, as shown in Table 6. In the same table note that the decrease in total annual precipitation in the Sierra region was 416.4 mm, while the increases in the other regions ranged from 17.9 mm for Pantanos to 201.8 mm for Centro: with anomalies that fluctuated between + 10.2% for Centro, + 1.1% for Pantanos and -12.1% for Sierra region. It is important to note that abnormalities in total annual precipitation observed in the study period, and those expected in the different scenarios for 2050 and 2100 horizons are consistent with the scenarios reported for Tabasco by ^{Conde and Gay (2008)}.

Table 6 Trend change in average annual total precipitation in the state of Tabasco and its regions during the period (1961-2010).