  As IPCC ARs stated, global warming is estimated based on the average from
1850 to 1900 (global average temperature of preindustrialization estimated from
relatively sparse observations). Given the impossibility of massive increasing
observation data in the early stages, accurately constraining this baseline has
become an unresolved issue. Here we developed a new statistical physical model
to quantify the contribution of external forcings to global warming as a
"deterministic trend" of the surface temperature series (instead of as
non-stationary processes that yield a stochastic trend) and constrained the
reconstruction of the early time series (1850-1880). We find that the existing
datasets slightly overestimated the temperature anomalies in this period, thus
the speed of global warming since pre-industrialization is still
underestimated.
