  The subject of this paper is an investigation of the nonlinear contributions
to the spectrum of the integrated Sachs-Wolfe (iSW) effect. We derive the
corrections to the iSW-auto spectrum and the iSW-tracer cross-spectrum
consistently to third order in perturbation theory and analyse the cumulative
signal-to-noise ratio for a cross-correlation between the PLANCK and EUCLID
data sets as a function of multipole order. We quantify the parameter
sensitivity and the statistical error bounds on the cosmological parameters
Omega_m, sigma_8, h, n_s and w from the linear iSW-effect and the systematical
parameter estimation bias due to the nonlinear corrections in a
Fisher-formalism, analysing the error budget in its dependence on multipole
order. Our results include: (i) the spectrum of the nonlinear iSW-effect can be
measured with 0.8\sigma statistical significance, (ii) nonlinear corrections
dominate the spectrum starting from l=100, (iii) an anticorrelation of the CMB
temperature with tracer density on high multipoles in the nonlinear regime,
(iv) a much weaker dependence of the nonlinear effect on the dark energy model
compared to the linear iSW-effect, (v) parameter estimation biases amount to
less than 0.1 sigma and weaker than other systematics.
