Spatial analysis of mangrove forest management for the reduction of air temperature and CO2 emission


 Surabaya is a coastal city that is still developing. Like other developing cities, Surabaya highly suppresses the mangrove forest for the residential, industry and other areas. Mangrove forest as a conservation land has an essential function as an oxygen source for people in Surabaya and it absorbs CO2. Mangrove forest functions to decrease the impact of global warming and maintain the coastal ecosystem sustainability. This research aimed to 1) Map the dynamics of temperature in Surabaya during the last twenty years (1996-2016) by using Remote Sensing and GIS. 2) Examine the ability of mangrove forest in absorbing CO2 and heat energy in decreasing the impact of global warming in Surabaya. The research results showed that 1) based on the analysis result of temperature surface extent, the temperature has changed since 1996 up to 2016. The trend of the temperature change can be classified into three surface temperatures which are low, moderate and high. The low temperature with the category of 210-300C had a different trend. Every year, the trend of the area extent of which surface temperature was highly categorized was 310-420C. The trend was highly increasing in the period of 2006-2016. Such condition showed that, in 2016, Surabaya had a high temperature increase. (2) There was a correlation between mangrove forest change and the temperature change; the content of CO2 in the mangrove, vegetation and water areas was reduced and it increased in the areas used for buildings such as factory, residential, road and other. The content of CO2 in the air of Surabaya showed the distribution grouping in the “high” and “extremely high” categories. The high category was 27.5% and the extremely high category was 67.5%. The sample point in each moderate and low category was around 25%.


Introduction
Some cities in Indonesia have a higher average value of surface temperature compared with their surrounding areas due to the high urban physical development. Such physical development includes: commercial, governmental, residential, industrial buildings, and so forth. In the contrary, the chart of vegetation zones showed that vegetation zones such as paddy fields, plantations, or forest conservation areas are in decline 1 .
There was a similar condition in Surabaya where an increasing temperature since 1994 to 2012 was reported. According to Bryan, 2013, the successive average temperatures recorded from 1994 until 2002 were 28.95 0 C, 27.07 0 C, 32.54 0 C and 38.23 0 C. The recorded temperature of 2006 and 2009 also increased, namely from 43.06 0 C became 44.30 0 C. However, the temperature recorded in 2012 declined to be 34.81 0 C. When the CO 2 concentration in the atmosphere reaches above 450 ppm, then the temperature would increase of more than 2 degree Celsius 2 .
The development of the Eastern part of Surabaya for the expansion of Juanda Airport, residential area, and industrial area resulted in mangrove forest preservation became vulnerable. Mangrove forest has a key role as a source of oxygen for Surabaya society, and it could absorb sun radiation in declining the impact of global warming. Mangrove forest contains a higher level of carbon than other kinds of forests on earth 3 . The mangrove ecosystem in Indonesia is able to absorb carbon content in the air of up to 67.7 MtCO per year. Rachmawati 4 stated that mangroves have a biomass potential of 108.66 and carbon content of 55.35. The high carbon content is influenced by the ability of mangroves to absorb carbon from its surrounds through a photosynthesis process known as the sequestration process 5 . During the photosynthesis process, the CO generated from the atmosphere will be bound by the vegetation and stored in the form of biomass, so that the CO content in the air will decline 6 .
The carbon assimilation ability of mangrove forest is 4 (four) level higher than other tropical forests all around the world 3 . The carbon content of mangroves is the highest in tropical areas, i.e. 1,023 Mg C/Ha (above-ground Carbon reserve). The absorbed carbon biomass varies in the age, species, morphology, and location of mangrove 7 .
Mangrove forests have a role in reducing the heat caused by climate change, especially through carbon sequestration 8 . Carbon sequestration by mangroves is more unique than that performed by other vegetation. Mangrove forests has an essential role in absorbing atmospheric carbon to control the environmental ecosystem in coastal areas 9 .
Plants can emit and absorb unique waves. Therefore, it can relate to waves beamed by other objects so that the vegetation can be distinguished from other objects 10 . The analysis can be done manually or by computer. The LanduseSim technique can be utilized for spatial simulation, including land-use simulation which can accommodate the bottom-up and top-down approaches 11 .
The remote sensing technology, besides forecasting or predicting the future, can present the data and facts of spatial issues happened in the past. Such spatial information can be gathered via spatiotemporal analysis which is superior in showing multi-level, multispectral, multitemporal images with larger spatial extent.
This research aimed (1) to map the dynamics of mangrove area and temperature in Surabaya during the last twenty years (1996-2016) by using Remote Sensing and Geographic Information System, and (2) to examine the ability of mangrove forest in absorbing CO 2 and heat energy in decreasing the impact of global warming in Surabaya.
Remote sensing is a science and art to obtain information about an object, area or symptom by analyzing the data recorded by the sensor without the need of a connection with the object, area or symptom 12 . The utilization of remote sensing can be found in various fields of study, namely mapping, agriculture, forestry, natural resource management, urban and regional planning, and other earth science 13,14 .
Landsat is an unmanned earth resources satellite which carries a non-photographic sensor. The most recently launched Landsat satellite on 15 April 1999 was Landsat 7 which carries an Enhanced Thematic Mapper Plus (ETM+) sensor. In brief, the characteristics of Landsat ETM+ are presented in Table 1 below:

Methods
This research was a survey research designed by using remote sensing and Geographic Information System (SIG). The remote sensing data in the forms of Landsat and Quickbird Satellite images were used to identify temporal changes of mangrove forests and temperature during the last 20-year period. The data were then integrated with the data resulted from the field measurement, and subsequently analyzed via SIG to identify the mangrove area distribution and temperature pattern in Surabaya. The remote sensing data obtained from Quickbird were used to study the existing land use in relation with the activities of the society around the mangrove area.
The objects of this research were changes occurred specifically in mangrove area in East Surabaya due to global warming. There were several Tools and Materials required in this research. The tools consisted of a computer, ENVI/ErMapper to analyze the Satellite images, SIG Software equipped with ArcGIS 10. The collected data were categorized into primary and secondary data. The primary data included the data of field temperature, the existing land use, and examination of mangrove forests based on the ability to absorb CO 2 and heat energy. The secondary data contained satellite images and maps collected through survey and documentation. The survey technique was intended to directly perform a field observation on the study objects, in this case, temperature, land use, and the amount of absorbed CO 2 and heat energy. Documentation was performed to collect the data in accordance with the actual spatial use and also as the authentic evidence.
The analysis of changes in mangrove forests was performed by NDVI (Normalized Difference Vegetation Index) method which is a standard method in comparing the level of vegetation of satellite data. The formula of NDVI is as follow: The analysis of mangrove ability in absorbing heat energy can be identified from the changes on temperature, especially in the mangrove area located in East Surabaya for the last 20 years. The thermal infrared channels in channel 6 of Landsat TM and ETM+ and in channel 10 and 11 in Landsat 8 were used to observe the temperature.
The analyses used to study the influence of changes in land cover on temperature were the Landsat 7 image analysis and a simple linear regression. The results of research obtained from the observation and field measurement were analyzed to be then explained or described. The Landsat 7 image analysis was conducted by converting the imagery data into temperature data. The converted imagery data were the pixel values in band 6 of the Landsat images called as digital number (DN). According to USGS (2003), there are two necessary steps to convert the imagery data into temperature data, namely: The final result obtained from the equation was in the form of temperature in a Kelvin (K) scale. In order to obtain temperature data in a Celsius ( 0 C) scale, the equation is changed into: A simple linear regression analysis is done when the types of the variables are free and bound. The changed area of land cover is a free variable (X) while the air temperature is a bound variable (Y). The regression analysis was performed by using SPSS 16 software for Windows.
c. The calculation of Biomass and Carbon of the Mangrove Ecosystem can be performed by the following formula 5 : Carbon and Biomass Potentials =

ni=1 = � � x mangrove area
Where: Yi = biomass potential per type I = the i type d. Spatial Modeling with LanduseSim The spatial modeling uses landuseSim to predict the changes on land use, temperature, and to determine the scenario of mangrove forest planning.

Temperature Changes in Surabaya
The imagery data used in this research were the Landsat 5 TM, Landsat 7 ETM+, and Landsat 8 OLI TIRS satellite images in the format of *.GeoTIFF and contained metadata in the format of *.MTL. The metadata provided information on the image recording time, solar elevation angle, image quality, cloud coverage area, and other important data which were relevant to the images. The imagery data utilized in this research are presented in Table 2. Several imagery data that were used in this research were a Landsat Level 1 Standard Data Product. The Level 1 Landsat data were imagery data result from the calibration and validation processes to measure and prevent sensory problems such as the atmospheric condition so that their quality can be improved by using some certain algorithms 4,15 .
The study of surface temperature in Surabaya in the last 2 decades was analyzed through Landsat TM, ETM+, and 8 images. The results of analysis can be seen in Table 3 below. Based on the results of an analysis of surface temperature area, trends of the temperature changes from 1996 until 2016 can be obtained. The trends can be classified into 3 temperature characteristics, i.e. low, moderate, and high. The low temperature comprises of the category of 11 0 -2 1 0 C having an areadevelopment trend from 1996-2016. It means that Surabaya experienced a positive temperature change for the low temperature category. The trend of changes of areas with low temperature shown in the following figure 1.   The surface temperature dynamics in Surabaya occurred in a period from 1996-2016 which were due to numerous factors. Some of the dominant factors were the population and their activities. The increasing number of the population went hand in hand with the increasing demand for living place or places for their activities, such as residential area, industrial area, trading site, and so forth. One example of threatened land use by such various activities is the land use for Mangrove forests.

Mangrove Area Changes in Surabaya
The temperature dynamics closely relate to changes in land function or land use. The land use that became under threat along with the growth of population in Surabaya was the land use for Mangrove Forests. The mangrove forest that functioned as a buffer zone was in a critical condition. The condition is illustrated in table 4 below. The trend of Mangrove area dynamics can be seen from Table 4  The currently existing mangrove forests can be found in rivers and estuaries or coastal areas. In a hinterland, mangroves grow along the rivers and embankments. Meanwhile, mangroves in estuaries or coastal areas are usually fertile and grow along the riversides. The spatial dynamics of the mangrove area in Wonorejo and the results of the survey are shown in Table 5 below. The spatial changes on mangrove area in Surabaya from 1996-2016 were related to the trend of temperature change. The temperature tends to be high in an open land or in a narrowing mangrove area. Such changes can be seen in table 6 below: Source: Data Analysist, 2017

Ability of Mangroves to absorb Carbon Dioxide (CO 2 ) from the Air in Surabaya
Based on the identification of CO 2 distribution in the air of Surabaya, the 40 samples taken were different in characters depended on the land use or land coverage. The lowest CO 2 distribution was 352 ppm and the highest was 587 ppm. The identification results of CO 2 distribution in Surabaya are presented in Table 7 below. Source: Data Analysist, 2017 The data collected from the survey shown in Table 7 showed the CO 2 content in the air of Surabaya was distributed in the categories of high and extremely high, i.e. 27.5 % and 67.5 %, respectively. There was 1 sample point in moderate and low categories with a percentage of 2.5 %. The CO 2 content in Surabaya was categorized as extremely high. It meant that Surabaya has already experienced a glasshouse effect that could be seen from the correlation between the temperature values and the CO 2 content in the air. The map of CO 2 spatial distribution in Surabaya shown in the following figure 5. The condition of CO 2 content is obtained on the land use as an estuary and the object with water characteristics, while the highest CO 2 content was generally obtained in developed area with the land uses of residential area, industrial area, roads, dry ponds, cemetery, and stations.
The above figure showed that, generally, the high temperature and high CO 2 content in Surabaya were found in developed areas. Usually, the CO 2 content is low in vegetation cover and waters. Even though the CO 2 content in mangrove area was categorized as moderate, but it was still lower than those in residential areas or other developed areas. It can be proven by a comparison of CO 2 content in mangrove forests and developed areas. The graph of CO 2 content changes in surabaya shown in the following figure 6.

Discussion
Based on the results of interpretation and survey on mangrove forest area, there is a correlation between land use changes and the increased surface temperature. The five sample points in the area of Wonorejo Mangrove Forest showed that there was a significant spatial effect of mangrove area conversion into developed area or open land. Such significant effect can be seen from the parameter of surface temperature changes observed based on Landsat images from 1996-2016 and the results of survey used to validate the temperature. It is in accordance with a study performed by Kristoufek 16 stated that there is a positive correlation between the increase of CO 2 emission and the global temperature.
The changes of mangrove area had unique stages. It was initiated with the land use conversion from mangrove area into ponds. Such conversion affected the temperature since the CO 2 which was initially absorbed by the mangrove can increase openly after the conversion.
The ponds located in the mangrove area had been explicitly owned by the society as private properties. Therefore, the people can change or sell their rights of ownership to other parties including housing developers.
Mangrove forests have an essential role for the city of Surabaya. Besides as a conservation area against erosion and sea abrasion, it also functions as a protector from global warming. Mangrove forests have a good ability to reduce the glasshouse gas, i.e. CO 2 17 . They are significant, global contributors for the low carbon cycle in the sea latitude, and glasshouse emission caused by tropical deforestation 9 .
The increasing concentration of carbon dioxide (CO 2 ) has a significant influence on plants. The selection of the type of plant determines whether or not it would endure and be able to adapt with the climate change 18 . It is in line with the selection of the type of mangrove to be developed that should consider its high ability to absorb CO 2 and to endure various kinds of sedimentation. Each type of mangrove is sensitive to the sedimentary condition and temperature rise 17 . A research conduct entitled "Estimation of Carbon Stock in Rhizophora Stylosa Tress at Camplong Beach, Sampang -Madura" concluded that the carbon content in the high tide zone was higher than that in the low tide zone 19 .
The spatial changes found in the mangrove forest area in Surabaya from 1996-2016 were related to the trend of temperature changes. Therefore, green areas should be included in spatial planning in order to control the global warming effects in Indonesia 20 .
The model of open green space management in reducing CO 2 should not only consider the green space in coastal areas but also along the commercial roads, railways, around the industrial area, residential area, and so on 21 . Since urban heat islands appeared frequently in areas with many activities, reforestation using reflective materials has positive impacts on global warming and urban heat island mitigation 22 .
The mangrove forests located along the east coast of Surabaya should be maintained as protection areas. Such mangrove forest management is a form of adaptation to and mitigation of global warming 8,23 .

Conclusion
There were changes on the mangrove area and temperature within two decades from 1996-2016. The mangrove area became larger in 2016. It was due to the distribution of mangrove to coastal areas because of sedimentation expansion. The temperature became higher due to the conversions of mangrove forests into ponds and developed areas such as airport, residential area, and industrial expansions.
The CO 2 content on waters, mangrove, and developed areas have different characteristics. The highest temperatures were found in residential and industrial areas and the lowest temperatures were found in the estuaries.