Global warming is the increase in the average temperature of the earth's near-surface air and oceans since the mid-twentieth century and its projected continuation. Current models predict a global warming of about 1.4 - 5.8° C from between 1990 to 2100. Human activities that contribute to climate change include the burning of fossil fuels and agriculture and land-use changes, like deforestation. These cause emissions or net increases of carbon dioxide (CO2), the main gas responsible for climate change, a s well as other 'greenhouse' gases. Climate change is often viewed as a phenomenon that will develop in the coming century, but its effects are already being seen. The two most discussed and prominent effects of climate change are increased temperature a nd carbon dioxide (CO2). In aquatic systems, oxygen levels, or dissolved and biologically available oxygen, are water temperature dependent where warmer water produces lower partial pressure of oxygen or pO2. High levels of aquatic CO2 concentrations (h ypercapnia) lead to a lower pH and can have effects on animal physiology such as a decrease in protein biosynthesis rates. Hypercapnia and temperature dependent changes in pO2 can lead to a lack of oxygen or hypoxic stress in aquatic vertebrate organisms. Because of the detrimental effects of decreased oxygen tension, organisms have developed a programmed response to this condition. Thus, a complex series of mechanisms associating temperature, CO2 and perflourinated pollutants with steroidogenesis or estr ogen signaling have been proposed. The implications of these types of associations can only be elucidated through systematic investigation and cod provides excellent model.