Missing write access for firebase CLI

I am attempting to install firebase CLI so I can write to the database using node.js while running on just the local server. However when I attempt to install firebase CLI using the following in the terminal:

npm install -g firebase-tools

I get this error:

Missing write access to /usr/local/lib/node_modules
/usr/local/lib

there is also the following error later on after attempting to install:

Error: EACCES: permission denied, access '/usr/local/lib/node_modules'
npm ERR!     at Error (native)
npm ERR!  { Error: EACCES: permission denied, access '/usr/local/lib/node_modules'

via dgelinas21