The Early 20th Century Warming (ETCW) in the northern high latitudes was comparable in magnitude to the present-day warming yet occurred at a time when the growth in atmospheric greenhouse gases was rising significantly less than in the last 40 years. The causes of ETCW remain a matter of debate. The key issue is to assess the contribution of internal variability and external natural and human impacts to this climate anomaly. This paper provides an overview of plausible mechanisms related to the early warming period that involve different factors of internal climate variability and external forcing. Based on the vast variety of related studies, it is difficult to attribute ETCW in the Arctic to any of major internal variability mechanisms or external forcings alone. Most likely it was caused by a combined effect of long-term natural climate variations in the North Atlantic and North Pacific with a contribution of the natural radiative forcing related to the reduced volcanic activity and variations of solar activity as well as growing greenhouse gases concentration in the atmosphere due to anthropogenic emissions.

Agriculture crop residue burning in tropics is an important source of atmospheric aerosols and monitoring their long-range transport is an important element in climate change studies. Synchronous measurements using micro-pulsed lidar, MICROTOPS-II sun photometer, multi-filter rotating shadow band radiometer (MFRSR) on aerosol optical depth and ground reaching solar irradiance were carried at an urban location in central region of India. Aerosol backscatter profiles obtained from micro-pulse lidar showed elevated aerosol layers up to similar to 3km on certain days during October 2007. Satellite data observations on aerosol properties suggested transport of particles from agriculture crop residue burning in Indo-Gangetic Plains over large regions. Radiative forcing of aerosols estimated from SBDART model with input information on aerosol chemical properties, aerosol optical depth and single scattering albedo and broadband solar irradiance measurements using MFRSR showed good correlation (R = 0.98). (C) 2008 Elsevier Ltd. All rights reserved.

Atmospheric aerosols are among the most variable components of the Earth's atmospheric environment important in general circulation models related to climate change. The presence of aerosols in the lower atmosphere affects primarily the incoming solar radiation by scattering and absorbing the solar radiation. Aerosols have significant impact on climate through their influence on cloud formation and on minor species concentrations. Studies on Aerosols with respect to temporal and spatial variations in different environments gains importance. Synchronous measurement of Aerosol Optical Depth (AOD), solar irradiance in different wavelength bands, aerosol particle size distribution measurements and Black Carbon (BC) aerosol mass concentration were made at urban area of Hyderabad, India as a part of ISRO-GBP initiative. The Julian day variation of AOD, Particulate Matter (PM) and BC showed higher values on certain days suggesting additional sources of aerosols over urban area of Hyderabad. In order to understand the additional sources of aerosol, daily satellite data sets of MODIS/DMSP-OLS were processed for forest fires over the Indian region. The higher values in black carbon aerosol mass concentration and aerosol optical depth correlated well with forest fires over the region. Radiative forcing estimated from synchronous measurements of AOD and ground reaching broadband solar irradiance. Ground AOD measurements correlated well with MODIS derived AOD at different wavelengths. Results are discussed in the paper.