Present-day climate forcing and response from black carbon in snow
Present-day climate forcing and response from black carbon in snow
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DOI：
10.1029/2006jd008003
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发表时间：
2007-06-05
影响因子：
4.4
通讯作者：
Rasch, Philip J.
中科院分区：
文献类型：
--
作者：
Flanner, Mark G.;Zender, Charles S.;Rasch, Philip J.
[1] 我们将雪、冰和气溶胶辐射 (SNICAR) 模型与具有预测碳气溶胶输送的大气环流模型相结合，以增进对雪中黑碳 (BC) 的气候强迫和响应的理解。在之前两项研究的基础上，我们考虑了年际变化的生物质燃烧碳黑排放、雪老化和雪融水的气溶胶清除。我们根据这些因素以及 BC 光学特性和积雪覆盖率来评估强制估计的不确定性。黑碳排放是最大的不确定性来源，其次是雪老化。积雪的老化速度决定了积雪的有效半径（r(e)），它直接控制积雪的反射率和BC引起的反照率变化的大小。对于合理的 r(e) 范围，BC 的反射率减少量变化三倍。低效的融水清除使疏水性杂质在融化过程中保持在表面附近并增强了强迫。应用强（1998）和弱（2001）北方火灾年的生物质燃烧BC排放清单，我们估计所有来源（化石燃料、生物燃料和生物质燃烧）的全球年平均BC/雪面辐射强迫分别为+0.054（0.007-0.13）和+0.049（0.007-0.12）W m（-2）。仅来自化石燃料+生物燃料来源的降雪强迫为+0.043 W m(-2)（仅来自化石燃料的强迫为+0.033 W m(-2)），表明人为对总强迫的贡献至少为80%。由于直接的BC/雪强迫，1998年全球陆地和海冰积雪分别吸收了0.60和0.23 W m(-2)。强迫力在融雪开始时达到最大，在当地春季引发强烈的雪反照率反馈。因此，BC/降雪强迫的“功效”比 CO2 强迫大三倍多。1998 年和 2001 年北纬 50 度以北的陆地融雪率在没有 BC 雪的控制模拟最大融化前一个月分别高出 28% 和 19%。根据气候反馈，当将 BC 纳入其中时，全球年平均 2 米气温升高 0.15 和 0.10 摄氏度。雪，而每年的北极 升温分别为 1.61 和 0.50 摄氏度。 1998 年高纬度气候响应比 2001 年更强，至少部分是由北方火灾造成的，1998 年北极强迫的 35% 生物质燃烧几乎全部由北方火灾造成。然而，在这个实验中，功效异常大，需要更多的研究来阐明北方火灾的作用，我们认为北方火灾具有最大的北极BC/强雪潜力 四月至六月期间。模型中雪中的 BC 浓度与来自不同地点的 23 个观测值相当吻合 (r = 0.78)，跨度接近 4 个数量级。我们预测中国东北地区的降雪浓度将超过 1000 ng g(-1)，足以使降雪反照率降低 0.13 以上。青藏高原瞬时强迫最大，春季部分地区超过20 W·m(-2)。 这些结果表明雪变暗是碳气溶胶气候强迫的重要组成部分。
[1] We apply our Snow, Ice, and Aerosol Radiative (SNICAR) model, coupled to a general circulation model with prognostic carbon aerosol transport, to improve understanding of climate forcing and response from black carbon (BC) in snow. Building on two previous studies, we account for interannually varying biomass burning BC emissions, snow aging, and aerosol scavenging by snow meltwater. We assess uncertainty in forcing estimates from these factors, as well as BC optical properties and snow cover fraction. BC emissions are the largest source of uncertainty, followed by snow aging. The rate of snow aging determines snowpack effective radius (r(e)), which directly controls snow reflectance and the magnitude of albedo change caused by BC. For a reasonable r(e) range, reflectance reduction from BC varies threefold. Inefficient meltwater scavenging keeps hydrophobic impurities near the surface during melt and enhances forcing. Applying biomass burning BC emission inventories for a strong (1998) and weak (2001) boreal fire year, we estimate global annual mean BC/snow surface radiative forcing from all sources ( fossil fuel, biofuel, and biomass burning) of +0.054 (0.007-0.13) and +0.049 (0.007-0.12) W m(-2), respectively. Snow forcing from only fossil fuel + biofuel sources is +0.043 W m(-2) (forcing from only fossil fuels is + 0.033 W m(-2)), suggesting that the anthropogenic contribution to total forcing is at least 80%. The 1998 global land and sea-ice snowpack absorbed 0.60 and 0.23 W m(-2), respectively, because of direct BC/snow forcing. The forcing is maximum coincidentally with snowmelt onset, triggering strong snow-albedo feedback in local springtime. Consequently, the "efficacy'' of BC/snow forcing is more than three times greater than forcing by CO2. The 1998 and 2001 land snowmelt rates north of 50 degrees N are 28% and 19% greater in the month preceding maximum melt of control simulations without BC in snow. With climate feedbacks, global annual mean 2-meter air temperature warms 0.15 and 0.10 degrees C, when BC is included in snow, whereas annual arctic warming is 1.61 and 0.50 degrees C-. Stronger high-latitude climate response in 1998 than 2001 is at least partially caused by boreal fires, which account for nearly all of the 35% biomass burning contribution to 1998 arctic forcing. Efficacy was anomalously large in this experiment, however, and more research is required to elucidate the role of boreal fires, which we suggest have maximum arctic BC/snow forcing potential during April-June. Model BC concentrations in snow agree reasonably well (r = 0.78) with a set of 23 observations from various locations, spanning nearly 4 orders of magnitude. We predict concentrations in excess of 1000 ng g(-1) for snow in northeast China, enough to lower snow albedo by more than 0.13. The greatest instantaneous forcing is over the Tibetan Plateau, exceeding 20 W m(-2) in some places during spring. These results indicate that snow darkening is an important component of carbon aerosol climate forcing.