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While a great deal of climate data have been gathered over the past hundred years, there remains a number of problems limiting our ability to fully utilize these data in reconstructing the climate of the past century. This is particularly true for research demanding high precision and/or detailed local or regional-scale climate analyses. In this review we consider our ability to quantify climate change with respect to near-surface air temperature (measured 1.25-2 m above ground), sea surface temperature, precipitation, snow cover, sea ice, and vegetation measured from space and the Earth's surface. Among the data issues we discuss are calibration, observing practices, urbanization, station changes, data representativeness, data access, and areal coverage. The diversity of measurements over the past century and the new monitoring system being introduced via space-based and surface-based platforms offer an unparalleled opportunity for global monitoring; but to quantify climate change, we must tackle such issues as changing retrieval algorithms, relatively short periods of record, satellite Earth location precision, incompatibility with previous conventional historical observations, calibration, and potentially overwhelming data volumes. A new specialty within the climate field is beginning to emerge to address these problems. Despite the litany of problems the instrumented climate record can tell us a great deal about the spatial distribution and secular trends in temperature and precipitation over many areas of the world. In the future a blend of many data types and observing systems will be necessary to better quantify climate change. These large data sets will have to be made accessible to scientists in such a way that allows them an opportunity to check the veracity of their hypotheses and predictions regarding climate change.