landsat70sNowChange

var landsat70s = ee.ImageCollection('LANDSAT/GLS1975');

var L81 = ee.ImageCollection("LANDSAT/LC08/C01/T1")
  .filterDate('2013-01-01','2013-12-31')
  .filter(ee.Filter.calendarRange(3,10,'month'));
var composite1 = ee.Algorithms.Landsat.simpleComposite({
  collection:L81,
  asFloat:true
});

var nir70s = landsat70s.select("30").mean();
var red70s = landsat70s.select("20").mean();
var ndvi70s = nir70s.subtract(red70s).divide(nir70s.add(red70s));

var nir1 = composite1.select("B5");
var red1 = composite1.select("B4");
var ndvi1 = nir1.subtract(red1).divide(nir1.add(red1));

var ndviChange = ndvi70s.subtract(ndvi1);

var ndvi = composite1.addBands(ndvi70s.rename('ndvi70s'))
  .addBands(ndvi1.rename('ndvi1'))
  .addBands(ndviChange.rename('ndviChange'));

Map.addLayer(ndvi, {bands: ["ndvi70s"], min:-0.5, max:1, opacity:1})
Map.addLayer(ndvi, {bands: ["ndvi1"], min:-0.5, max:1, opacity:1})
Map.addLayer(ndvi, {bands: ["ndviChange"], min:-0.5, max:0.5, opacity:1})

var ndvi = ndvi.visualize({
  bands: ['ndviChange'],
  min:-2, max:2
});

Export.image.toDrive({
  image: ndvi,
  description: 'ndviChange',
  crs: 'EPSG:3857',
  scale: 30
});