By ivie • May 30th, 2007 at 6:35 pm • Category: Bumi, News

Tahun 2006, Greenland mengalami hari-hari mencairnya salju pada ketinggian yang lebih tinggi dibanding ketinggian rata-rata selama 18 tahun. Hasil pengamatan harian menunjukkan mencairnya salju di lapisan es Greenland mengalami peningkatan setiap harinya.

Monitoring terhadap pelelehan saju di lapisan es Greenland secara harian dilakukan dengan Special Sensor Microwave Imaging radiometer (SSM/I) yang berada di pesawat ruang angkasa Defense Meteorological Satellite Program. Sensor akan mengukur sinyal elektromagnetik yang dipancarkan lapisan es dan mendeteksi lelehan salju yang terjadi lebih dari 10 hari lebih lama dari rata-rata yang terjadi pada area tertentu di Greenland.

Dengan adanya hasil pengamatan satelit secara periodik memberikan data dan informasi yang akan membantu para peneliti untuk mengetahui kecepatan alir glacier, banyaknya air dari salju yang mencair dan bergabung dengan lautan disekitarnya, juga untuk mengetahui seberapa banyak radiasi Matahari yang akan dipantulkan kembali ke atmosfer.

Salju kering dan basah memang terlihat sama jika dilihat untuk pertama kalinya. Tapi salju yang basah dan salju yang mengalami pembekuan kembali, memiliki tingkat penyerapan radiasi sinar Matahari yang lebih tinggi, dan hanya memantulkan 50-60 persen ke atmosfer. Sedangkan salju kering, memantulkan kembali 85 % radiasi Matahari. Dengan kata lain, salju yang meleleh akan menyerap 3-4 kali energi yang sama dibanding salju kering. Ini tentu akan memberi pengaruh yang besar pada persediaan energi di Bumi.

Mencairnya salju di Greenland memberi pengaruh yang sangat besar terhadap luas lapisan es yang terus berkurang dan terhadap tinggi dan dalam lautan diseluruh dunia. Sebagian air yang dihasilkan dari salju yang mencair juga akan mengalir kedalam glacier melalui patahan-patahan dan alur lubang vertikal (moulin), kemudian mencapai lapisan batuan dibawahnya dan melubrikasi (meminyaki, mencairkan) lapisan es diatasnya.

Pengamatan dan studi yang dilakukan sebelumnya oleh Jay Zwally dan Waleed Abdalati dari NASA Goddard menunjukkan, air yang mencair pada musim panas pada dasar lapisan es bisa meningkatkan gerak es dan menyebabkan terjadinya peningkatan level lautan (tinggi dan dalamnya) dengan sangat cepat. Fenomena ini akan mempercepat terjadinya pemanasan global.

Gambar skematik permukaan glacial yang mengilustrasikan bagaimana moulins mentransport air ke dasar glacier. credit : NASA.

sumber : NASA

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Overview of Earth Radiation Budget

Most input of the Earth energy is received from the Sun. The solar energy is short-wave radiation. Although the Earth also receives electromagnetic energy from the other bodies in space, it’s negligible, compared with solar energy [Table 1]. The incident solar energy (shortwave) may be reflected and absorbed by the Earth’s surface or the atmosphere. And Earth’s surface and atmosphere also emit the radiation (longwave).

This figure is cited from NASA homepage. If you want to visit there, click the picture. The Earth Radiation Budget is the balance between incoming energy from the sun and the outgoing longwave (thermal) and reflected shortwave energy from the Earth.

Solar Energy

The radiant solar Energy is from nuclear energy and the temperature of the Sun is 6000K. The spectrum of the solar radiation received at the top of the atmosphere is well approximated by the spectrum of a blackbody having a surface temperature of about 6000K. Thus Sun may be considered as a blackbody.

The solar energy reaching the Earth is traditionally quantified as the solar constant which is the annual average solar irradiance received outside the Earth’s atmosphere or surface normal to the incident radiation at the Earth’s mean distance from the Sun (about 1370 W/m2). The actual solar irradiance from the Sun. The actual solar irradince varies by 3.4% from the solar constant during the year due to the eccentricity of Earth’s orbit about the Sun.

Surface radiation budget

The Surface Radiation Budget is divided into downward shortwave radiation, reflected shortwave radiation, downward longwave radiation, upward longwave radiation, net radiation. They are dominated by cloud.

The downward shortwave radiation may be reflected to the space absorbed in the atmosphere and absorbed at the ground.

Esun = AEsun + Eatm + (1-Asfc)Esfc where Esun is the incident solar radiation, A is albedo of TOA (Top of atmophere), Eatm is the energy of absorbed in atmosphere, Esfc is downward irradiance at surface and Asfc is surface albedo. This equation implicity includes scattering and multiple reflectors between the surface and clouds. Usually the incident solar radiation and TOA albedo is measured by satellite measurement.(ERBE) The reflected solar radiation is the product of surface albedo and the downward solar radiation, the surface albedo should be determined. One method to estimate surface albedo is the minimum albedo technique. Because few locations are likely to be cloud-covered for an entire month, the minimum albedo is likely to be represent the clear-sky albedo. It can be calculated from narrow band AVHRR observations. The downward longwave radiation is mostly from the atmosphere. It depends on the temperature and moisture of the atmosphere. The water vapor and other gases, aerosols absorb some solar energy and emit some longwave radiation energy computation of downward longwave radiation from the atmosphere is difficult, even when the distributions of water vapor, carbon dioxide, cloudiness, and temperature are measured. Some satellite measurements like TOVS estimates downward longwave radiation. Little longwave radiation is reflected by the surface: natural surface emission is dominant. It is also difficlut to measure and define the surface temperature especially vegetation surface. To combine the above four components makes the calculation of net radiation at the surface. This is not accurate because the errors in each accumulate. So it is developed the research to use some satellite measurements-NOAA, GOES etc.

Earth Radiation Budget Experiment

In 1978, an international team of Scietists was selected to design and develop of ERBE. Dr. Bruce Barkstern was the ERBE principal Investigator. This team developed two types of instruments :

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