Firn data compilation reveals widespread decrease of firn air content in western Greenland
oleh: B. Vandecrux, B. Vandecrux, M. MacFerrin, H. Machguth, H. Machguth, W. T. Colgan, D. van As, A. Heilig, C. M. Stevens, C. Charalampidis, R. S. Fausto, E. M. Morris, E. Mosley-Thompson, L. Koenig, L. N. Montgomery, C. Miège, S. B. Simonsen, T. Ingeman-Nielsen, J. E. Box
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2019-03-01 |
Deskripsi
<p>A porous layer of multi-year snow known as firn covers the Greenland-ice-sheet interior. The firn layer buffers the ice-sheet contribution to sea-level rise by retaining a fraction of summer melt as liquid water and refrozen ice. In this study we quantify the Greenland ice-sheet firn air content (FAC), an indicator of meltwater retention capacity, based on 360 point observations. We quantify FAC in both the uppermost 10 m and the entire firn column before interpolating FAC over the entire ice-sheet firn area as an empirical function of long-term mean air temperature (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mover accent="true"><mrow><msub><mi>T</mi><mtext>a</mtext></msub></mrow><mo mathvariant="normal">‾</mo></mover></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="12pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="3f34f2e28fd131f1588d0a4a19d8cca4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-845-2019-ie00001.svg" width="12pt" height="16pt" src="tc-13-845-2019-ie00001.png"/></svg:svg></span></span>) and net snow accumulation (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mover accent="true"><mi>c</mi><mo mathvariant="normal">˙</mo></mover><mo mathvariant="normal">‾</mo></mover><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="12pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="2467b5ce9e85c5b99553be3356b76c30"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-845-2019-ie00002.svg" width="12pt" height="15pt" src="tc-13-845-2019-ie00002.png"/></svg:svg></span></span>. We estimate a total ice-sheet-wide FAC of <span class="inline-formula">26 800±1840</span> km<span class="inline-formula"><sup>3</sup></span>, of which <span class="inline-formula">6500±450</span> km<span class="inline-formula"><sup>3</sup></span> resides within the uppermost 10 m of firn, for the 2010–2017 period. In the dry snow area (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mrow><msub><mi>T</mi><mtext>a</mtext></msub></mrow><mo mathvariant="normal">‾</mo></mover><mo>≤</mo><mo>-</mo><mn mathvariant="normal">19</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="45pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="6fe53f25472c89b27849cfe687f9f7b3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-845-2019-ie00003.svg" width="45pt" height="16pt" src="tc-13-845-2019-ie00003.png"/></svg:svg></span></span> <span class="inline-formula"><sup>∘</sup></span>C), FAC has not changed significantly since 1953. In the low-accumulation percolation area (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mrow><msub><mi>T</mi><mtext>a</mtext></msub></mrow><mo mathvariant="normal">‾</mo></mover><mo>></mo><mo>-</mo><mn mathvariant="normal">19</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="45pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="42f9350ebdfedbd2e9ccdfba4e695aaa"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-845-2019-ie00004.svg" width="45pt" height="16pt" src="tc-13-845-2019-ie00004.png"/></svg:svg></span></span> <span class="inline-formula"><sup>∘</sup></span>C and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><mover accent="true"><mover accent="true"><mi>c</mi><mo mathvariant="normal">˙</mo></mover><mo mathvariant="normal">‾</mo></mover><mo>≤</mo><mn mathvariant="normal">600</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="39pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3745663665b56693e2ecbf0a733ea7ad"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="tc-13-845-2019-ie00005.svg" width="39pt" height="14pt" src="tc-13-845-2019-ie00005.png"/></svg:svg></span></span> mm w.e. yr<span class="inline-formula"><sup>−1</sup>)</span>, FAC has decreased by <span class="inline-formula">23±16</span> % between 1998–2008 and 2010–2017. This reflects a loss of firn retention capacity of between <span class="inline-formula">150±100</span> Gt and <span class="inline-formula">540±440</span> Gt, respectively, from the top 10 m and entire firn column. The top 10 m FACs simulated by three regional climate models (HIRHAM5, RACMO2.3p2, and MARv3.9) agree within 12 % with observations. However, model biases in the total FAC and marked regional differences highlight the need for caution when using models to quantify the current and future FAC and firn retention capacity.</p>