def calc_cbar(c_0, c_curr):
    """
    Calculate the co2 concentration average between the historical co2 and
    current co2 concentrations
    
    Parameters
    ----------
    c_0 : float
        Historical co2 concentration, in ppm
    c_curr : float
        Current co2 concentration, in ppm
        
    Return
    ------
    c_bar : float
        Averaged concentration, in W m^-2 ppm^-1
    """
    c_bar = 0.5 * (c_0 + c_curr)
    return c_bar