Is there a way to get a global minimum and maximum value for an Earth Engine Image?

Question

I want to determine the 'global' minimum and maximum value for a calculated variable in Google Earth Engine. Essentially, my goal is to use a global geometry in reduceRegion(). However, Earth Engine complains when I use an unbounded geometry (image.geometry()) as my region argument. Is there a way to get the range of a calculated image in GEE?

Here is my code:

////////////////////////////////////////////////////////
// Get global min and max of SPEI from TerraClim data///
////////////////////////////////////////////////////////
// Get dataset
var terraClim = ee.ImageCollection('IDAHO_EPSCOR/TERRACLIMATE').filterDate('2017-10-01', '2018-09-30');
terraClim = terraClim.select('pr', 'pet');

// Calculate SPEI from the precipitation and potential evapotranspiration bands
var spei = terraClim.map(function(img){
  img = ee.Image(img);
  var pr = img.select("pr");
  var pet = img.select("pet");
  var spei = img.expression('pr - pet', {pr:pr,pet:pet});
  return img.select()
    //.addBands(pr.rename('pr'))
    //.addBands(pet.rename('pet'))
    .addBands(spei.rename('spei'))
    .set('system:time_start', img.get('system:time_start'));
});

Map.addLayer(spei.first(), 
{min:-2000, max:100, palette:['white','red', 'orange', 'yellow', 'brown', 'green', 'blue', 'purple']}, 'spei');

// Get global min and max values of SPEI using reduceRegion
var spei_min = spei.first().reduceRegion({
  reducer: ee.Reducer.min(), 
  geometry: spei.geometry(), 
  bestEffort: true, 
  });

var spei_max = spei.first().reduceRegion({
  reducer: ee.Reducer.max(), 
  geometry: spei.geometry(), 
  bestEffort: true, 
  });
  
print('Global minimum SPEI:', spei_min);
print('Global maximum SPEI:', spei_max);

Here is the error I get when I try to print the resulting dictionaries:

Image.reduceRegion: Provide 'geometry' parameter when aggregating over an unbounded image.

Link to my code in code editor: https://code.earthengine.google.com/32957edb7eb0b4797b503dc7605a10b5

Answer 1

Use ee.Geometry.BBox(-180, -90, 180, 90) as the geometry argument.