Sharing values through namespaces

My question is probably stupid, but I can't share a value through namespaces.

namespace AceEngine
{
    namespace Graphics
    {
        namespace Interface
        {
            void drawDebugScreen()
            {
                // I want to access AceEngine::System::Version from here.
            }
        }
    }

    namespace System
    {
        string Version("DEBUG");
    }
}

How can I access this string?

EDIT:

ae.cpp

#include "stdafx.h"
#include "sha256.h"
#include <iostream>
#include <string>
using std::cout;
using std::cin;
using std::endl;
using std::getline;
using std::string;

namespace AceEngine
{
    namespace Graphics
    {
        namespace Interface
        {
            void drawDebugScreen()
            {
                cout << "Version: " << AceEngine::System::Version << endl;
            }
            class Window{};
        }
    }
    namespace System
    {
        class User{};
        void pause(){cin.get();}
        extern string Version("DEBUG");
    }
}

ae.h

#pragma once
#include "stdafx.h"
#include <string>
using std::string;

namespace AceEngine
{
    namespace Graphics
    {
        namespace Interface
        {
            void drawDebugScreen();
            class Window{};
        }
    }

    namespace System
    {
        class User{};
        void pause();
        extern string Version;
    }
}

I removed the useless parts (I left some classes to show there are stuff in the namespaces and it's not useless)