...I would replace multi-and bilateral development assistance programs with programs of debt restructuring. A troika from, say, Bolivia, Burkina Faso and Bangladesh would arrive every few months to Washington, London and Brussels to see if the ecological debts are paid off according to schedule and if not: no next slice of vital resources from South to North.
...I would rename it a large cooperative bank . One that is Too big to fail - with the consequence that climate change can not be allowed to assume catastrophic proportions. Thus, citizens and businesses will get Greenhouse gas (GHG) quotas. They start from the present average GHG consumption per capita or business but go down fast. For the personal quota they evolve rapidly towards the eight tons lifestyle: what every world citizen can use per year.
...then you would have a third credit card (next to the classic card and the carbon credit card): for your local currency. Regional currencies would flourish complementary to for example the euro and they organize a significant part of the local and regional market in services and products.
...then every citizen gets a basic income: a sum that in the region of residence can cover the real minimum: food, clothes, shelter, education and health. The market arranges everything above that basic income - but with some corrections: the pay gap between the highest paid and lowest paid in a company should never be more than 10. Anything above a modest maximum wage is taxed at 100%. Those earning more will get a sweet thank you card for their generous contributions to the public interest, with a personal signature of the Prime Minister.
... I would introduce a financial transaction tax and a ban on short selling: which boils down to making profit by spreading bad rumors about others. Savings and investment banks are separated and bankers who reap billions through cheating with interest levels or other financial hocus-pocus do not just go behind bars. No. We use their expertise in finding loopholes through making them part of the cell to find financial fraud. In the first year of internment they will get food stamps depending on the amount of detected fraud. From the second year and when good behavior is shown they can switch to a system with ankle strap.
...then Environmental criminals would also have a problem. The newly established International Environmental Court - in collaboration with Interpol - will arrest for example the top of Chevron, who after twenty years and several convictions refuse to pay 30,000 Ecuadorians for damages their company has caused. The bosses of the British Trafigura would - as an alternative penalty - receive 5 years of work in healthcare centers in Abidjan. Schmidheiny, a billionaire convicted in an asbestos trial in Italy, would be obliged to remove fasbestos plates from houses in the first ten years and then move on to ten years of work in palliative care - if he has not ended up there by then.
... then the education curriculum gets some updates with as required reading in every sixth secondary: Collapse (Jared Diamond), Nineteen Eighty-Four (George Orwell) and The Age of absurdity (Michael Foley). Video material from The Story of Stuff is also raised and every year, every student goes on a week long survival trip. In fact: Government Ministers also go at least one week each year in nature. Compulsory leave outside the city. Their assistant takes up his or her affairs and the leader in question only takes with it what he or she can carry on the back. Alone or with family.
... then advertisements of unsustainable products are banned - such as printers that fail after x number of prints. The people that design such products are being sentenced to five years of community service on the garbage dumps of Delhi, Gambia or China. They may choose.
Finally, I would swing the ax to researchers who are engaged in a search for geoengineering or planet B. Or at least in their grants.
Here is what I would do when I would rule the world. My submission to a column comptetion was selected by the jury as one of the 5 nominees BUT NOW IT IS OPEN FOR PUBLIC VOTING. You can read all five columns 
