Part of the problem is that daily data have more complex issues than monthly data. For example, while homogeneity assessments need to be done to account for the effects of changing from one type of instrument shelter to another, the proper daily adjustment depends not only on whether it is summer or winter, but whether the day was clear or overcast and windy or calm. This is particularly true when one wants to use the adjusted data to assess changes in extremes. Because of this complexity, the issues involved in homogeneity adjustments for daily data are just beginning to be addressed. Daily data are also far less available than monthly means. "Global" datasets have large portions of South America, Central America, Africa and Southwest Asia lacking long-term daily observations. Even the assessment of what constitutes "long-term" data is different. Daily observations are often considered long-term if they are several decades long while monthly observations are widely available on century time scales.
For the IPCC's Fourth Assessment Report, the climate research community intends to better quantify the observed changes in extreme events. To accomplish this considerable efforts will need to be made to address problems with daily data. Fortunately, a variety of efforts are already underway to improve the situation. The Global Climate Observing System Surface Network is slowly growing, regional climate change workshops are providing climate change assessments and climate change indices derived from daily data for under assessed areas of the world, and individual research projects are building regional and global databases. Yet much remains to be done. A great deal of data still needs to be digitized. Political concerns need to be overcome so more data can be exchanged. And additional work is required to develop robust homogeneity adjustments to daily data.