  Observations of high-redshift quasars at z>6 imply that supermassive black
holes (SMBHs) with masses over a billion solar masses were in place less than 1
Gyr after the Big Bang. If these SMBHs assembled from "seed" BHs left behind by
the first stars, then they must have accreted gas at close to the Eddington
limit during a large fraction (>50%) of the time. A generic problem with this
scenario, however, is that the mass density in million-solar-mass SMBHs at z=6
already exceeds the locally observed SMBH mass density by several orders of
magnitude; in order to avoid this overproduction, BH seed formation and growth
must become significantly less efficient in less massive protogalaxies, while
proceeding uninterrupted in the most massive galaxies that formed first. Using
Monte-Carlo realizations of the merger and growth history of BHs, we show that
X-rays from the earliest accreting BHs can provide such a feedback mechanism.
Our calculations paint a self-consistent picture of black-hole-made climate
change, in which the first miniquasars---among them the ancestors of the z>6
quasar SMBHs---globally warm the IGM and suppress the formation and growth of
subsequent generations of BHs. We present two specific models with global
miniquasar feedback that provide excellent agreement with recent estimates of
the z=6 SMBH mass function. For each of these models, we estimate the rate of
BH mergers at z>6 that could be detected by the proposed gravitational-wave
observatory eLISA/NGO.
