Complex sentence: Currently the price of fossil fuels companies shares is calculated under the assumption that all of the company's fossil fuel reserves will be consumed and so the true costs of carbon dioxide in intensifying global warming is not taken into account in a company's stock market valuation.
Split into simpler sentences: Currently the price of fossil fuels companies shares is calculated under the assumption that all fossil fuel reserves will be consumed. It claims that the true costs of carbon dioxide in intensifying global warming is not taken into account in a company's stock market valuation.