Climate feedbacks are negative, not positive.
Without positive climate feedbacks, the nominal net warming due to a doubling of carbon dioxide is 0.7 C. The postulated positive feedback effects are:
1. Water vapor increases as temperature increases, leading to further warming due to increased thermal radiation trapping.
2. Decreased snow and ice cover, leading to more warming due to increased absorption of solar radiation because of the darker surface.
3. Decreased (or sometimes increased) cloud cover leading to more (or sometimes less) warming.
The 0.7 C warming is thus increased to the IPCC range of 1.5 to 4.5 C from these feedbacks.
Each feedback hasn’t been measured yet and the cloud feedback is very uncertain. The water vapor feedback has been measured and is very weak. The sum total of the feedbacks has been examined in two recent papers and found to be negative. The papers are:
Douglass, David H., Eric G. Blackman, and Robert S. Knox, 2004. Temperature response of Earth to the Annual Solar Irradiance Cycle. Physics Letters A Vol. 323, No 3-4, pp. 315-322. Working document online <http://www.arxiv.org/ftp/astro-ph/papers/0403/0403271.pdf>
Abstract: We directly determine the sensitivity and time delay of Earth's surface temperature response to annual solar irradiance variations from 60 years of data. A two-layer energy balance model is developed to interpret the results. Explaining both the resulting low sensitivity and time delay of 1-2 months requires negative feedback.
Karner, O., 2002: On non-stationarity and anti-persistency in global temperature series. J. Geophys. Res. 107, D20. (See http://www.aai.ee/~olavi/2001JD002024u.pdf).
Abstract: Statistical analysis is carried out for satellite-based global daily tropospheric and
stratospheric temperature anomaly and solar irradiance data sets. Behavior of the series
appears to be nonstationary with stationary daily increments. Estimating long-range
dependence between the increments reveals a remarkable difference between the two
temperature series. Global average tropospheric temperature anomaly behaves similarly to
the solar irradiance anomaly. Their daily increments show antipersistency for scales longer
than 2 months. The property points at a cumulative negative feedback in the Earth climate
system governing the tropospheric variability during the last 22 years. The result
emphasizes a dominating role of the solar irradiance variability in variations of the
tropospheric temperature and gives no support to the theory of anthropogenic climate
change. The global average stratospheric temperature anomaly proceeds like a 1-dim
random walk at least up to 11 years, allowing good presentation by means of the
autoregressive integrated moving average (ARIMA) models for monthly series.
Karner states: “The revealed antipersistence in the lower tropospheric temperature increments does not support the science of global warming developed by IPCC . Negative long-range correlation of the increments during last 22 years means that negative feedback has been dominating in the Earth climate system during that period. The result is opposite to suggestion of Mitchell  about domination of a positive cumulative feedback after a forced temperature change. Dominating negative feedback also shows that the period for CO2 induced climate change has not started during the last 22 years. Increasing concentration of greenhouse gases in the Earth atmosphere appeared to produce too weak forcing in order to dominate in the Earth climate system.”
Karner finds a Hurst exponent of 0.27 +/-0.04 for tropospheric temperatures. From an online primer by Ian Kaplan <http://www.bearcave.com/misl/misl_tech/wavelets/hurst/>, the following meaning is giving to this number:
“The values of the Hurst exponent range between 0 and 1. A value of 0.5 indicates a true random walk (a Brownian time series). In a random walk there is no correlation between any element and a future element. A Hurst exponent value H, 0.5 < H < 1 indicates "persistent behavior" (e.g., a positive autocorrelation). If there is an increase from time step ti-1 to ti there will probably be an increase from ti to ti+1. The same is true of decreases, where a decrease will tend to follow a decrease. A Hurst exponent value 0 < H < 0.5 will exist for a time series with "anti-persistent behavior" (or negative autocorrelation). Here an increase will tend to be followed by a decrease. Or a decrease will be followed by an increase. This behavior is sometimes called "mean reversion".”
Keys conclusions from these studies:
1. The climate system has a net negative feedback, opposite to what the claim of a positive feedback made by the IPCC.
2. Consequently, “increasing concentration of greenhouse gases in the Earth atmosphere appeared to produce too weak forcing in order to dominate in the Earth climate system."