random - Addition of signal variance and noise variance

Can I add two variances directly? I have my noise variance $\sigma_{noise}=-10 \ \mathrm{dB}$ and signal variance, $\sigma_{signal}=40 \ \mathrm{dB}$. So, can I say the total variance is $\sigma_{total}=30 \ \mathrm{dB}$?

I will use this total variance to generate random variables using a normal distribution with $\mu =0$ and $\sigma = \sigma_{total}$.