Blackett Laboratory, Imperial College London, London SW7 2AZ, UK.
Nature. 2010 Oct 7;467(7316):696-9. doi: 10.1038/nature09426.
The thermal structure and composition of the atmosphere is determined fundamentally by the incoming solar irradiance. Radiation at ultraviolet wavelengths dissociates atmospheric molecules, initiating chains of chemical reactions-specifically those producing stratospheric ozone-and providing the major source of heating for the middle atmosphere, while radiation at visible and near-infrared wavelengths mainly reaches and warms the lower atmosphere and the Earth's surface. Thus the spectral composition of solar radiation is crucial in determining atmospheric structure, as well as surface temperature, and it follows that the response of the atmosphere to variations in solar irradiance depends on the spectrum. Daily measurements of the solar spectrum between 0.2 µm and 2.4 µm, made by the Spectral Irradiance Monitor (SIM) instrument on the Solar Radiation and Climate Experiment (SORCE) satellite since April 2004, have revealed that over this declining phase of the solar cycle there was a four to six times larger decline in ultraviolet than would have been predicted on the basis of our previous understanding. This reduction was partially compensated in the total solar output by an increase in radiation at visible wavelengths. Here we show that these spectral changes appear to have led to a significant decline from 2004 to 2007 in stratospheric ozone below an altitude of 45 km, with an increase above this altitude. Our results, simulated with a radiative-photochemical model, are consistent with contemporaneous measurements of ozone from the Aura-MLS satellite, although the short time period makes precise attribution to solar effects difficult. We also show, using the SIM data, that solar radiative forcing of surface climate is out of phase with solar activity. Currently there is insufficient observational evidence to validate the spectral variations observed by SIM, or to fully characterize other solar cycles, but our findings raise the possibility that the effects of solar variability on temperature throughout the atmosphere may be contrary to current expectations.
大气的热结构和组成从根本上取决于入射的太阳辐照度。紫外线波长的辐射会使大气分子离解，引发化学反应链——特别是那些产生平流层臭氧的反应，并为中层大气提供主要的加热源，而可见光和近红外波长的辐射主要到达并加热低层大气和地球表面。因此，太阳辐射的光谱组成对于确定大气结构以及地表温度至关重要，因此，大气对太阳辐照度变化的响应取决于光谱。自 2004 年 4 月以来，太阳辐射与气候实验（SORCE）卫星上的光谱辐照度监测仪（SIM）仪器每天在 0.2 µm 至 2.4 µm 之间测量太阳光谱，结果表明，在太阳活动的这个下降阶段，紫外线的下降幅度比根据我们以前的理解预测的要大四到六倍。这种减少在总太阳输出中部分被可见光波长辐射的增加所补偿。在这里，我们表明，这些光谱变化似乎导致 2004 年至 2007 年间 45 km 以下平流层臭氧的显著减少，而在这个高度以上则有所增加。我们的结果与 Aura-MLS 卫星同期臭氧测量结果一致，使用辐射光化学模型进行模拟，尽管时间较短，难以精确归因于太阳效应。我们还使用 SIM 数据表明，地表气候的太阳辐射强迫与太阳活动不同步。目前，还没有足够的观测证据来验证 SIM 观察到的光谱变化，或充分描述其他太阳周期，但我们的发现提出了一种可能性，即太阳变化对整个大气温度的影响可能与当前的预期相反。
